Structural diversity among multinary pnictide oxides: a minireview focused on semiconducting and superconducting heteroanionic materials
-
Sviatoslav Baranets
,
Gregory M. Darone
Abstract
Incorporating different anions with varied ionic sizes and charges is a rapidly growing approach to bring out unusual physical properties among various classes of solid-state materials, pnictides and chalcogenides in particular. This minireview is focused on hetero-anionic materials based on the pnictogens, which have been demonstrated to offer an impressive diversity of crystal chemistry and electronic structures. In addition, many pnictide oxides or oxypnictides, over the course of the last decade, have been shown to exhibit a broad spectrum of superconducting, magnetic, and semiconducting properties. However, the structural diversity of the mixed-anion materials is far greater than the several known structure types, or their variants, of the well-known layered superconductive materials. Therefore, with this treatise, we aim to provide a comprehensive overview of the crystal chemistry of pnictide oxides by recounting almost 40 different structures of such ternary and multinary compounds. In addition to the structural aspects, we also highlight some of the challenges associated with the synthesis, and briefly summarize reported, to date, physical properties of this remarkable class of solids.
1 Introduction
The simultaneous presence of oxygen and pnictogen (Pn), i.e., electronegative element of group 15 (Pn = P, As, Sb, Bi), occurs in numerous ternary and multinary compounds. The vast majority of these are classic inorganic salts, such as phosphates, phosphonates, arsenates, etc. These compounds can be understood by recognizing the formation of distinct oxoanions with Pn–O bonding; the character of the bonding is largely polar covalent, with the polarity being associated with the partial electron transfer from the less electronegative pnictogen to the more electronegative oxygen. Therefore, in most simple terms, the pnictogen atoms are characterized as cations in such salts, with typical oxidation states of +3 or +5. Less-common positive oxidation states are also possible, for example the +4 in the hypophosphate P2O6 4− ion featuring homoatomic P–P bonding. From a solid-state chemistry point of view, rationalizing the bonding in such compounds with the concepts of pnictide cations and oxide anions is supported, since the electronic band structures are typically characterized by distinct energy separation between the occupied O2− p states and empty Pn states; therefore, such materials are usually wide-bandgap semiconductors (or rather, insulators).
In contrast to these single-anion oxide-based pnictate phases, compounds with electropositive elements such as metals (yet with no distinct Pn–O bonding) contain another type of anion, Pn 3−, which is found in pnictides [1]. Such solid-state phases with pnictogen and oxygen as anions are referred to as pnictide oxides or oxypnictides. This class of compounds is a part of the intensively developing large family of heteroanionic materials, such as pnictide chalcogenides, pnictide halides, and chalcogenide halides, to name a few [2]. A unique mixed-anion composition results in the formation of structures different from those typical for pnictides or oxides, synergizing structural arrangements from both classes. Consequently, a structural uniqueness results in interesting physical properties, making mixed-anion compounds favorable candidates for catalysis, energy conversion, and electronic materials, including those with infrared nonlinear optical properties, semiconducting properties, and superconductivity [3], [4], [5], [6].
The comprehensive investigation of oxypnictide materials by the solid-state chemistry community began in recent years only. The first comprehensive review on pnictide oxides was published in 1995 by Kauzlarich and co-authors [7], summarizing the data available at that time for around 30 known compounds. The 2008 discovery of superconductivity in the iron-arsenides and related pnictide oxides noticeably increased the interest in this field [8], [9], [10], [11], [12]. Over the last years, many other oxypnictide phases have been reported, including many crystallizing in their own structure types; there have been a number of detailed studies describing structures and physical properties as well.
Structural and compositional diversity in pnictide oxides materials can be traced to the semiconducting Zintl pnictides [13, 14], many of which are compositionally akin to various oxypnictides. Whereas a coordination environment of the oxide O2− anion is usually different than for the Pn 3− anion, the simultaneous presence of both noticeably enriches the structural diversity. Interestingly, the Pn:O ratio in the typical pnictide oxides usually exceeds 1, as highlighted in this minireview (with a few exceptions). Commonly, oxygen-rich compounds are derivatives of the oxyacids, and the oxoanions, as mentioned already feature Pn–O bondіng characteristics which do not belong to the realm of the mixed-anion compounds discussed herein.
On this note, here, we need to bring the attention to the fact that in oxypnictides, the consideration of both oxide O2− and pnictide Pn 3− anions implies an increased average charge transfer level (i.e., high degree of ionicity of the bonding). This also suggests that the valence electrons from the electropositive element(s) will be “fully” donated to the electropositive pnictogen and oxygen atoms, which will use them to form anionic or polyanionic species, satisfying the octet rule. Of course, the relatively small amount of oxygen atoms in the structure also means that the degree of polarity of the bonding will only be slightly higher than in related pnictide materials’ structures. Thus, the main features of the bonding within the structures of the oxypnictides, in analogy to the Zintl phases [13, 14], can be described by the Zintl–Klemm concept [15]; oxypnictides can therefore be considered as charge-balanced narrow bandgap semiconductors or semimetals.
In the following paragraphs, we recount the structural chemistry and properties of many known ternary and multinary pnictide oxides. This minireview is not exhaustive, although it covers nearly two dozen well-studied layered structures, vide infra, as well as several unique examples that crystallize in their own structure types. Novel heteroanionic solid-state materials are being discovered and studied at a very fast pace and by summarizing and highlighting the recent advances and trends, we hope that this article will serve the scientific community and will promote additional theoretical and experimental research in this field.
2 Synthesis
The synthetic approaches, methodologies, and conditions for preparing oxypnictide phases are similar to the techniques used for the synthesis of other Zintl pnictides. A more elaborate overview of the typical synthetic methods and their applicability for the synthesis of Zintl pnictides (and oxypnictides) can be found elsewhere [13, 14, 16]. Herein, only several aspects of the synthetic work of particular relevance to oxypnictides will be outlined.
Since compounds of interest to the condensed matter community generally need to be prepared in bulk form, we discuss only reactions aimed at obtaining single-crystalline or polycrystalline samples in gram-quantities. Such experiments usually require a combination of high temperatures and inert atmosphere. Oxygen often becomes an inadvertent reagent at such conditions, and in fact, many oxypnictides have originally been detected as side products of reactions targeting the synthesis of pnictide phases. While it is complicated to unambiguously determine the source of the latter, partial oxidation of air-sensitive starting materials, such as alkaline-earth of rare-earth metals, or outgassing of silica tubes were believed to be culprits. Alumina crucibles used for flux reaction should also be considered as a possible source of oxygen. It was recently shown that the reaction between the crucible and specific fluxes might even promote tuning the oxygen content in the melt [17].
The limited content and unadvertised origin of oxygen in many known pnictide oxide materials complicated its detection in the crystal structures. Several early works had misidentified ternary oxypnictides as binary pnictides primarily because of the weak scattering origin of oxygen atoms and limited sensitivity of instruments. An excellent example was the discovery of Ca2 Pn (Pn = Sb, Bi) phases with the tetragonal La2Sb structure type [18], which later had to be revised and reformulated to be Ca4 Pn 2O with the anti-K2NiF4 structure [19, 20]. It is also well established that some intermetallic phases, such as Mn5Si3-type, Th2Zn17-type or Ti8Bi9-type can be stabilized by non-metal impurities, such as boron, carbon, nitrogen, and oxygen which are reasonably considered as anionic species [21], [22], [23], [24]. Such compositions can also be considered as oxypnictides in cases where oxygen served as a “stabilizing agent”. In the case of Ca4 Pn 2O [19, 20], electronic structure calculations readily provide clues pointing at the electronic instability of Ca2 Pn, suggesting that modern computational chemistry might become a powerful tool for screening the structural databases for potential omissions.
The unadvertised origin of oxygen in the initial discoveries usually resulted in low yields of pnictide oxides. Therefore the formation of phase-pure ternary and multinary oxypnictide materials in quantitative amounts often requires a direct reaction between the stoichiometric amounts of metals and metal/pnictogen oxides, peroxides, or superoxides. Such reactions also require an inert atmosphere in order to exclude unmetered oxygen, although conditions for the synthesis vary depending on the studied system and are often unique for different compounds. In some cases, reactions have to be performed in Nb and Ta tubes, yet the reactivity of several pnictogens, such as As and P with Nb and Ta tubes, has to be considered [25], [26], [27]. Therefore, using bound pnictogens, such as binary metal pnictides or sesquioxides of pnictogens as less-reactive intermediate products, is one of the possible solutions. Proper choice of starting materials benefits reaction kinetics and phase purity. For instance, solid-state metathesis reaction between LaOCl and NaFeAs ternary phases yields in the synthesis of LaFeAsO pnictide oxide with noticeably improved phase purity [28].
The synthesis of oxypnictides usually requires relatively high temperatures, as high as 1000 °C or above. However, the formation of single crystals has been detected even at relatively low temperatures of ca. 650 °C, as was shown for the ternary Ba3Sb4O phase [29]. Proper homogenization, phase purity, and densification can be achieved by grinding together manually or using a ball-mill followed by pressing pellets and annealing or by spark-plasma sintering (SPS) [30].
The discovery of pnictide oxides with novel structures remains mostly of serendipitous nature. Recent advances in computational materials discovery are often limited by predicting compounds within known structure types. However, further development of machine-learning-driven screening techniques and approaches may lead to the discovery of new mixed-anion semiconducting materials [31]. While structural reports reviewed herein often lack property characterization, further first-principle calculations will benefit comprehensive characterization and potentially reveal promising properties and applications. A good illustration of this approach is the recent prediction of thermoelectric properties of Ca4 Pn 2O (Pn = Sb, Bi) phases [32], which for decades have remained as a laboratory curiosity.
3 Crystal chemistry and properties of the oxypnictide compounds
The main section of this review describes crystal structures and properties for known ternary and multinary heteroanionic pnictide oxides. Basic crystallographic data, i.e., the structure type, the space group symbol, and lattice parameters, are summarized in Tables 1 and 2. Data are arranged in the order of compositional complexity, starting from ternary compounds, and grouped by structure types. Several compounds of the same structure type are organized in the order of increasing atomic masses of pnictogen.
Crystallographic data for ternary pnictide oxides.
| Compound | Structure type | Space group | Pearson symbol | a, Å | b, Å/β (°) | c, Å | ICSD collection code | References |
|---|---|---|---|---|---|---|---|---|
| Ca4P2O | K2NiF4 | I4/mmm | tI14 | 4.497(1) | a | 15.103(5) | 402952 | [33] |
| Ca4As2O | K2NiF4 | I4/mmm | tI14 | 4.537(1) | a | 15.449(4) | 68203 | [34] |
| Ca4Sb2O | K2NiF4 | I4/mmm | tI14 | 4.677(2) | a | 16.342(7) | 16353 | [19] |
| Ca4Bi2O | K2NiF4 | I4/mmm | tI14 | 4.7192(16) | a | 16.505(9) | 416137 | [20] |
| Sr4P2O | K2NiF4 | I4/mmm | tI14 | 4.794(1) | a | 15.985(4) | 33903 | [35] |
| Sr4As2O | K2NiF4 | I4/mmm | tI14 | 4.831(1) | a | 16.364(4) | 33904 | [35] |
| Sr4Bi2O | K2NiF4 | I4/mmm | tI14 | 4.9948(7) | a | 17.548(4) | 710070 | [36] |
| Ba4As2O | K2NiF4 | I4/mmm | tI14 | 5.1246(4) | a | 17.336(1) | 33905 | [35] |
| Ba4Sb2O | K2NiF4 | I4/mmm | tI14 | 5.116(10) | a | 17.946(32) | 402284 | [37] |
| Ba4Bi2O | K2NiF4 | I4/mmm | tI14 | 5.2662(7) | a | 18.625(4) | 710071 | [36] |
| Eu4As2O | K2NiF4 | I4/mmm | tI14 | 4.7924(4) | a | 16.1933(9) | 1222 | [38] |
| Eu4Sb2O | K2NiF4 | I4/mmm | tI14 | 4.919(1) | a | 17.124(4) | 402953 | [39] |
| Eu4Bi2O | K2NiF4 | I4/mmm | tI14 | 4.9540(3) | a | 17.367(3) | 402954 | [40] |
| Sm4Bi2O | K2NiF4 | I4/mmm | tI14 | 4.9228(4) | a | 17.558(2) | 423294 | [41] |
| Yb4As2O | K2NiF4 | I4/mmm | tI14 | 4.562(4) | a | 15.44(1) | 402951 | [42] |
| Ba4P2O | La2CuO4 | Cmca | oS28 | 7.335(1) | 7.164(1) | 16.732(3) | 33906 | [35] |
| La2SbO2 | anti-ThCr2Si2 | I4/mmm | tI10 | 4.0674(6) | a | 13.705(3) | 262301 | [43] |
| Ce2SbO2 | anti-ThCr2Si2 | I4/mmm | tI10 | 4.0167(3) | a | 13.710(2) | 419608 | [44] |
| Pr2SbO2 | anti-ThCr2Si2 | I4/mmm | tI10 | 3.9957(3) | a | 13.580(2) | 419607 | [44] |
| Nd2SbO2 | anti-ThCr2Si2 | I4/mmm | tI10 | 3.9648(6) | a | 13.555(3) | 262302 | [43] |
| Sm2SbO2 | anti-ThCr2Si2 | I4/mmm | tI10 | 3.9220(6) | a | 13.323(2) | 262303 | [43] |
| Gd2SbO2 | anti-ThCr2Si2 | I4/mmm | tI10 | 3.8900(6) | a | 13.277(3) | 262304 | [43] |
| Ho2SbO2 | anti-ThCr2Si2 | I4/mmm | tI10 | 3.8279(5) | a | 13.039(3) | 262305 | [43] |
| Er2SbO2 | anti-ThCr2Si2 | I4/mmm | tI10 | 3.8151(5) | a | 13.007(3) | 262306 | [43] |
| Y2BiO2 | anti-ThCr2Si2 | I4/mmm | tI10 | 3.8734(1) | a | 13.2469(5) | 261432 | [45] |
| La2BiO2 | anti-ThCr2Si2 | I4/mmm | tI10 | 4.08483(6) | a | 13.9866(2) | 422645 | [45] |
| Ce2BiO2 | anti-ThCr2Si2 | I4/mmm | tI10 | 4.0369(5) | a | 13.746(2) | 419606 | [44] |
| Pr2BiO2 | anti-ThCr2Si2 | I4/mmm | tI10 | 4.0149(1) | a | 13.7017(6) | 419609 | [44] |
| Nd2BiO2 | anti-ThCr2Si2 | I4/mmm | tI10 | 3.99258(5) | a | 13.6663(2) | 422646 | [45] |
| Sm2BiO2 | anti-ThCr2Si2 | I4/mmm | tI10 | 3.95296(6) | a | 13.5083(3) | 261433 | [45] |
| Gd2BiO2 | anti-ThCr2Si2 | I4/mmm | tI10 | 3.9181(1) | a | 13.4246(5) | 261435 | [45] |
| Tb2O2Bi | anti-ThCr2Si2 | I4/mmm | tI10 | 3.8962(6) | a | 13.317(3) | 423916 | [46] |
| Dy2BiO2 | anti-ThCr2Si2 | I4/mmm | tI10 | 3.8761(3) | a | 13.233(2) | 423915 | [46] |
| Ho2BiO2 | anti-ThCr2Si2 | I4/mmm | tI10 | 3.86212(9) | a | 13.2262(4) | 261434 | [45] |
| Er2BiO2 | anti-ThCr2Si2 | I4/mmm | tI10 | 3.84531(1) | a | 13.1513(4) | 422647 | [45] |
| La3SbO3 | Own | C2/m | mS28 | 13.856(3) | 4.1050(8)/118.55(3) | 12.343(3) | 380456 | [47] |
| Sm3SbO3 | La3SbO3 | C2/m | mS28 | 13.332(2) | 3.9501(4)/118.310(11) | 11.9187(18) | 380455 | [47] |
| Gd3SbO3 | La3SbO3 | C2/m | mS28 | 13.272(3) | 3.8971(2)/118.40(3) | 11.878(2) | 380460 | [47] |
| Ho3SbO3 | La3SbO3 | C2/m | mS28 | 13.014(3) | 3.8218(7)/118.213(14) | 11.679(2) | 380458 | [47] |
| Gd3BiO3 | La3SbO3 | C2/m | mS28 | 13.444(3) | 3.8449(8)/118.31(3) | 12.033(2) | 429459 | [48] |
| Sm8Sb3O8 | Own | C2/m | mS38 | 13.462(3) | 3.8929(8)/106.77(3) | 15.230(3) | 380454 | [47] |
| Gd8Sb3O8 | Sm8Sb3O8 | C2/m | mS38 | 13.394(3) | 3.8191(8)/107.05(3) | 15.104(3) | 380459 | [47] |
| Ho8Sb3O8 | Sm8Sb3O8 | C2/m | mS38 | 13.1368(11) | 3.7609(2)/106.958(7) | 14.8700(13) | 380457 | [47] |
| Gd8Bi3O8 | Sm8Sb3O8 | C2/m | mS38 | 13.3939(3) | 3.87360(10)/107.031(2) | 15.1160(3) | 428036 | [48] |
| La9Sb5O5 | Own | P4/n | tP38 | 10.415(1) | a | 9.340(1) | 414019 | [49] |
| Ce9Sb5O5 | La9Sb5O5 | P4/n | tP38 | 10.297(1) | a | 9.228(1) | 414018 | [49] |
| Pr9Sb5O5 | La9Sb5O5 | P4/n | tP38 | 10.2203(3) | a | 9.1508(3) | 241209 | [50] |
| Sm9Sb5O5 | La9Sb5O5 | P4/n | tP38 | 10.0341(4) | a | 8.9839(3) | 241207 | [50] |
| Tb9Sb5O5 | La9Sb5O5 | P4/n | tP38 | 9.8618(12) | a | 8.838(2) | 409978 | [51] |
| Dy9Sb5O5 | La9Sb5O5 | P4/n | tP38 | 9.8389(3) | a | 8.7986(3) | 241208 | [50] |
| Ba3Sb2O | Own | Pbam | oP24 | 12.4228(11) | 12.630(3) | 5.101(3) | 280592 | [52] |
| Ba3Sb4O | Own | P2/c | mP32 | 6.9342(17) | 12.807(3)/112.385(10) | 12.941(3) | 415032 | [29] |
| Ti8BiO7 | Own | Cmmm | oS32 | 7.8473(4) | 16.8295(10) | 3.0256(2) | 239415 | [53] |
Crystallographic data for multinary pnictide oxides.
| Compound | Structure type | Space group | Pearson symbol | a, Å | b, Å/β (°) | c, Å | ICSD collection code | Reference |
|---|---|---|---|---|---|---|---|---|
| (Ca,Pr)9Sb5O5 | La9Sb5O5 | P4/n | tP38 | 10.2234(14) | a | 9.1379(18) | 429690 | [68] |
| (Ca,Sm)9Sb5O5 | La9Sb5O5 | P4/n | tP38 | 10.0668(14) | a | 9.0111(18) | 429691 | [68] |
| (Ca,Gd)9Sb5O5 | La9Sb5O5 | P4/n | tP38 | 9.9921(14) | a | 8.9758(18) | 429692 | [68] |
| (Ca,Gd)9Bi5O5 | La9Sb5O5 | P4/n | tP38 | 10.1018(14) | a | 9.0270(18) | 429693 | [68] |
| (Ca,Dy)9Bi5O5 | La9Sb5O5 | P4/n | tP38 | 9.99960(10) | a | 8.9341(2) | 492694 | [68] |
| (La,Dy)3SbO3 | Li3AuO3 | P42/mnm | tP28 | 11.9473(17) | a | 3.9263(8) | 425139 | [63] |
| (La,Ho)3SbO3 | Li3AuO3 | P42/mnm | tP28 | 11.9203(17) | a | 3.9057(8) | 425140 | [63] |
| (Ce,Ho)3SbO3 | Li3AuO3 | P42/mnm | tP28 | 11.8802(17) | a | 3.8906(8) | 425138 | [63] |
| CaCe3SbO4 | Ti5Te4 | I4/m | tI18 | 9.6084(14) | a | 4.0148(8) | 426810 | [58] |
| CaPr3SbO4 | Ti5Te4 | I4/m | tI18 | 9.5632(14) | a | 3.9754(8) | 426811 | [58] |
| CaNd3SbO4 | Ti5Te4 | I4/m | tI18 | 9.5541(14) | a | 3.9507(8) | 426812 | [58] |
| CaSm3SbO4 | Ti5Te4 | I4/m | tI18 | 9.4414(130 | a | 3.8960(8) | 426813 | [58] |
| CaGd3SbO4 | Ti5Te4 | I4/m | tI18 | 9.4205(13) | a | 3.8388(8) | 426814 | [58] |
| Ca2Pr8Sb3O10 | In6Sn8S19 | C2/m | mS46 | 13.651(3) | 3.9229(8)/93.54(3) | 17.628(4) | 427079 | [58] |
| Ca2Nd8Sb3O10 | In6Sn8S19 | C2/m | mS46 | 13.605(3) | 3.8884(8)/93.47(3) | 17.594(4) | 426815 | [58] |
| Ca2Sm8Sb3O10 | In6Sn8S19 | C2/m | mS46 | 13.462(3) | 3.8429(8)/93.62(3) | 17.458(4) | 426816 | [58] |
| Ca2Gd8Sb3O10 | In6Sn8S19 | C2/m | mS46 | 13.386(3) | 3.7985(8)/93.59(3) | 17.368(4) | 426817 | [58] |
| Ca2Tb8Sb3O10 | In6Sn8S19 | C2/m | mS46 | 13.307(3) | 3.7791(8)/93.67(3) | 17.271(4) | 426818 | [58] |
| KBa11P7O2 | Own | Fddd | oF168 | 10.699(1) | 15.143(2) | 31.646(4) | 85421 | [70] |
| KBa11As7O2 | KBa11P7O2 | Fddd | oF168 | 10.878(2) | 15.423(2) | 32.324(4) | 85422 | [70] |
| KBa4Sb3O | Sr5Pb3F | I4/mcm | tI36 | 8.824(1) | a | 16.5936(16) | 410747 | [71] |
| RbBa4Sb3O | Sr5Pb3F | I4/mcm | tI36 | 8.937(4) | a | 16.427(9) | 415036 | [29] |
| KBa4Bi3O | Sr5Pb3F | I4/mcm | tI36 | 8.9675(14) | a | 16.664(3) | 409487 | [72] |
| α-CeZnPO | ZrCuSiAs | P4/nmm | tP8 | 4.038(1) | a | 8.896(1) | 85777 | [73] |
| β-CeZnPO | NdZnPO |
|
hR8 | 4.012(4) | a | 31.21(3) | 416475 | [74] |
| β-PrZnPO | NdZnPO |
|
hR8 | 3.990(2) | a | 31.08(1) | 416476 | [74] |
| β-NdZnPO | Own |
|
hR8 | 3.976(1) | a | 30.955(5) | 85776 | [73] |
| β-SmZnPO | NdZnPO |
|
hR8 | 3.946(2) | a | 30.71(1) | 418525 | [75] |
| β-GdZnPO | NdZnPO |
|
hR8 | 3.922(2) | a | 30.56(1) | 418524 | [75] |
| β-DyZnPO | NdZnPO |
|
hR8 | 3.8933(6) | a | 30.305(4) | 418526 | [75] |
| β-YZnPO | NdZnPO |
|
hR8 | 3.885(2) | a | 30.32(1) | 418523 | [75] |
| RbSm2Fe4As4O2 | KCa2Fe4As4F2 | I4/mmm | tI26 | 3.9209(2) | a | 31.381(2) | –a | [76] |
| Ba2Cd3–x Bi3O | Own | I4/mmm | tI36 | 4.7396(4) | a | 43.601(7) | 261122 | [77] |
| Th2Ni3–iP3O | Own | P4/nmm | tP18 | 3.9462(4) | a | 17.232(3) | 83373 | [78] |
| U2Cu2As3O | Own | P4/nmm | tP16 | 3.9111(2) | a | 17.916(4) | 75132 | [79] |
| Sm2CuAs3O | Hf2CuGe4 | Pnma | oP28 | 34.469(1) | 3.948(1) | 3.984(1) | 174368 | [80] |
| La3Cu4P4O2 | Own | I4/mmm | tI26 | 4.033(1) | a | 26.765(8) | 84699 | [81] |
| Pr3Cu4P4O2–x | La3Cu4P4O2 | I4/mmm | tI26 | 3.978(1) | a | 26.587(5) | 412191 | [82] |
| Sm3Cu4P4O2–x | La3Cu4P4O2 | I4/mmm | tI26 | 3.928(1) | a | 26.436(5) | 412192 | [82] |
| La3Cu4As4O2 | La3Cu4P4O2 | I4/mmm | tI26 | 4.130(1) | a | 27.486(1) | –a | [83] |
| Ce3Cu4As4O2 | La3Cu4P4O2 | I4/mmm | tI26 | 4.07733(10) | a | 27.4146(9) | 259394 | [84] |
| La5Cu4P4O4Cl2 | La5Cu4As4O4Cl2 | I4/mmm | tI36 | 4.0752(5) | a | 40.568(7) | 430390 | [85] |
| La5Cu4As4O4Cl2 | Own | I4/mmm | tI36 | 4.1346(7) | a | 41.44(1) | 421137 | [83] |
| Ba2Mn3P2O2 | Sr2Mn3As2O2 | I4/mmm | tI18 | 4.202(1) | a | 19.406 | –a | [86] |
| Sr2Mn3As2O2 | Own | I4/mmm | tI18 | 4.16(1) | a | 18.84(4) | 32010 | [87] |
| Ba2Mn3As2O2 | Sr2Mn3As2O2 | I4/mmm | tI18 | 4.248(5) | a | 19.77(3) | 32011 | [87] |
| Sr2Mn3Sb2O2 | Sr2Mn3As2O2 | I4/mmm | tI18 | 4.262(4) | a | 20.11(2) | 32012 | [87] |
| Ba2Mn3Sb2O2 | Sr2Mn3As2O2 | I4/mmm | tI18 | 4.367(5) | a | 20.78(2) | 32013 | [87] |
| Sr2Mn3Bi2O2 | Sr2Mn3As2O2 | I4/mmm | tI18 | 4.28(1) | a | 20.55(5) | 32014 | [87] |
| Ba2Zn3As2O2 | Sr2Mn3As2O2 | I4/mmm | tI18 | 4.206(1) | a | 19.665(6) | 67998 | [88] |
| Sr2Cr3As2O2 | Sr2Mn3As2O2 | I4/mmm | tI18 | 4.00800(2) | a | 18.8214(1) | 28102 | [89] |
| Ba2Cr3As2O2 | Sr2Mn3As2O2 | I4/mmm | tI18 | 4.05506(2) | a | 20.5637(1) | –a | [89] |
| Ba2MnZn2As2O2 | Sr2Mn3As2O2 | I4/mmm | tI18 | 4.24257(8) | a | 19.5087(7) | 85659 | [90] |
| Sr2MnZn2As2O2 | Sr2Mn3As2O2 | I4/mmm | tI18 | 4.12298(3) | a | 18.6587(2) | 92875 | [91] |
| Sr2Mn2CuAs2O2 | Sr2Mn3As2O2 | I4/mmm | tI18 | 4.0833(2) | a | 18.5919(9) | 169513 | [92] |
| Sr2Mn2CrAs2O2 | Sr2Mn3As2O2 | I4/mmm | tI18 | 4.09354(10) | a | 19.0646(6) | 13782 | [93] |
| Sr2VFeAsO3 | Sr2CuGaSO3 | P4/nmm | tP16 | 3.9296 | a | 15.6732 | 165984 | [94] |
| Ba2ScFeAsO3 | Sr2CuGaSO3 | P4/nmm | tP16 | 4.1266(5) | a | 16.8001(15) | 420654 | [95] |
| Sr2CrFeAsO3 | Sr2CuGaSO3 | P4/nmm | tP16 | 3.91122(4) | a | 15.7905(3) | 420653 | [95] |
| Sr3Sc2Fe2As2O5 | Sr3Fe2O5Cu2S2 | I4/mmm | tI28 | 4.0781(1) | a | 26.8386(5) | 260449 | [96] |
| Eu3Sc2Fe2As2O5 | Sr3Fe2O5Cu2S2 | I4/mmm | tI28 | 4.06400(6) | a | 26.4694(7) | 429704 | [97] |
| Na2Ti2As2O | Own | I4/mmm | tI14 | 4.070(2) | a | 15.288(4) | –a | [98] |
| Na2Ti2Sb2O | Na2Ti2As2O | I4/mmm | tI14 | 4.144(1) | a | 16.5610(10) | 69648 | [98] |
| Sc4Yb4Sb4O | Na2Ti2As2O | I4/mmm | tI14 | 4.2721(3) | a | 16.0869(15) | 427132 | [99] |
| BaTi2As2O | Own | P4/mmm | tP6 | 4.047(3) | a | 7.275(4) | 169074 | [100] |
| BaTi2Sb2O | BaTi2As2O | P4/mmm | tP6 | 4.1055(2) | a | 8.0712(4) | 430061 | [101] |
| BaTi2Bi2O | BaTi2As2O | P4/mmm | tP6 | 4.12316(4) | a | 8.3447(1) | 430063 | [102] |
| Ba2Ti2Fe2As4O | Own | I4/mmm | tI22 | 4.0276(1) | a | 27.3441(4) | 263018 | [103] |
| Ba2Ti2Cr2As4O | Ba2Ti2Fe2As4O | I4/mmm | tI22 | 4.0391(1) | a | 27.8474(3) | 257022 | [104] |
| Ba2Mn2As2O | Own | C2/m | mS14 | 7.493(4) | 4.196(1)/96.17(3) | 10.352(3) | 75454 | [105] |
| Ba2Mn2Sb2O | Own | P63/mmc | hP14 | 4.71(1) | a | 20.04(2) | 16360 | [106] |
| Ba2Mn2Bi2O | Ba2Mn2Sb2O | P63/mmc | hP14 | 4.803(5) | a | 20.097(10) | 16361 | [106] |
| Ba5Cd2Sb4O2 | Own | C2/m | mS26 | 17.247(7) | 4.9279(18)/132.558(4) | 12.240(5) | 423458 | [107] |
| Nd10Au3As8O10 | Own | I4/m | tI62 | 9.1503(14) | a | 14.579(3) | 428964 | [108] |
| Sm10Au3As8O10 | Nd10Au3As8O10 | I4/m | tI62 | 9.0836(10) | a | 14.352(2) | 428962 | [108] |
| LT-Nd10Pd3As8O10 | Own | C2/c | mS124 | 12.709(1) | 14.690(1)/135 | 17.973(1) | –a | [109] |
| HT-Nd10Pd3As8O10 | Nd10Au3As8O10 | I4/m | tI62 | 9.005(1) | a | 14.735(2) | –a | [109] |
| Sm10Pd3As8O10 | Nd10Au3As8O10 | I4/m | tI62 | 8.948(1) | a | 14.523(4) | –a | [109] |
| Eu5Zn2As5O | Eu5Cd2Sb5F | Cmcm | oS52 | 4.3457(11) | 20.897(5) | 13.571(3) | 2004823 | [110] |
| Eu5Cd2As5O | Eu5Cd2Sb5F | Cmcm | oS52 | 4.4597(9) | 21.112(4) | 13.848(3) | 2004824 | [110] |
| Ba5Cd2Sb5O1–x | Eu5Cd2Sb5F | Cmcm | oS52 | 4.9582(5) | 23.346(2) | 15.2796(14) | 421814 | [111] |
| Eu5Cd2Sb5O | Eu5Cd2Sb5F | Cmcm | oS52 | 4.7088(5) | 21.965(2) | 14.5982(15) | 261244 | [112] |
| La2AuP2O | La2AuP2O | C2/m | mS24 | 15.373(3) | 4.2739(8)/131.02(1) | 10.092(2) | 423802 | [113] |
| Ce2AuP2O | La2AuP2O | C2/m | mS24 | 15.152(4) | 4.2463(8)/130.90(2) | 9.992(2) | 425401 | [114] |
| Pr2AuP2O | La2AuP2O | C2/m | mS24 | 15.036(4) | 4.2277(8)/130.88(2) | 9.930(2) | 425402 | [114] |
| Nd2AuP2O | La2AuP2O | C2/m | mS24 | 15.0187(5) | 4.2085(5)/131.12(1) | 9.903(3) | 425403 | [114] |
| Ce9Au5–x As8O6 | Own | Pnnm | oP116 | 13.2164(6) | 40.730(3) | 4.2396(2) | 431599 | [115] |
| Pr9Au5–x As8O6 | Ce9Au5–xAs8O6 | Pnnm | oP116 | 13.1501(4) | 40.5287(8) | 4.2068(1) | 431600 | [115] |
-
aICSD code unavailable.
Crystal structures of the discussed pnictide oxides have been comprehensively studied with the aid of single-crystal X-ray diffraction or neutron powder diffraction in most cases. With the growing demand for data accessibility, we suggested that ICSD collection codes (where possible) will be helpful for readers by providing convenient access to the structure of interest for further investigation. Atomic coordinates and unit cell parameters used for the figures were taken from the ICSD entries.
3.1 Coordination environments and building blocks
Apparent similarities in chemistries of the pnictide oxides and Zintl pnictides imply the existence of common structural features for both classes of compounds with regard to the Pn 3− anion. Indeed, numerous Zintl (poly)anions, such as dimers (Figure 1a), square nets (Figure 1b), or chains (Figure 1c) made of pnictide atoms, exist for both families. More complex anions, such as [MPn 3] with trigonal planar coordination (Figure 1d), [MPn 4] with tetrahedral or square-planar coordination (Figure 1e and f), [MPn 5] with square-pyramidal coordination (Figure 1g) (M = transition metal; Pn = pnictogen), are also well-known building blocks for both pnictide and pnictides oxides. Structural chemistry of the oxide part in oxypnictides is abundant and represented by the O-centered trigonal planes [OAE 3] (Figure 1d), tetrahedra or square-planes [OAE 4], [OM 4] (Figure 1e and f), square pyramids [OAE 5] (Figure 1g), and octahedra [OAE 6] (Figure 1h) (AE = alkaline-earth or rare-earth metal; M = transition metal).
![Figure 1:
Selected building blocks and structural fragments typical for pnictide oxide phases. (a) [Sb–Sb]4– dimers observed in the Ba3Sb2O and Ba5Cd2Sb5O structures; (b) polypnictide square nets
[
As
]
n
∞
2
${}_{\infty }{}^{2}{[\text{As}]}_{n}$
observed the U2Cu2As3O structure; (c) polypnictide chains
[
Sb
]
n
∞
1
${}_{\infty }{}^{1}{[\text{Sb}]}_{n}$
observed in the Ba3Sb4O structure; (d) trigonal-planar coordination typical for oxygen [OAE
3] and coinage metals [MAn
3] (M = Cu, Ag, Au); (e) square-planar coordination observed for oxygen [OAE
4], [OM
4] and transition metals [MAn
4] (M = Ti, Ni, Pd, Au); (f) O-centered [OAE
4] and M-centered [MAn
4] tetrahedron – the most common structural fragment in oxypnictides (M = transition metal); (g) pyramidal coordination typical for oxygen [OAE
5] and early transition metals [MAn
5] (M = Sc, Ti, V); (h) octahedral coordination typical for oxygen [OAE
6] and titanium [TiAn
6]; (i) fluorite type [M
2
Pn
2] layers (M = divalent transition metals, such as M = Ti, Cr, Mn, Fe, Co, Ni, Zn, Cd); AE = alkali, alkaline-earth, rare-earth metal; An = O, P, As, Sb Bi. Here and after AE, M, Pn, and O atoms are represented by the same size spheres regardless of their chemical identities and bonds drawn between them.](/document/doi/10.1515/zkri-2021-2079/asset/graphic/j_zkri-2021-2079_fig_001.jpg)
Selected building blocks and structural fragments typical for pnictide oxide phases. (a) [Sb–Sb]4– dimers observed in the Ba3Sb2O and Ba5Cd2Sb5O structures; (b) polypnictide square nets
The listed structural polyhedra serve as primary building blocks, which can be merged with three main types of connectivity: corner-sharing, edge-sharing, and face-sharing. Resulting arrangements form infinite 1D, 2D, and 3D substructures, while compounds with layers are the most studied among pnictide oxides. A distinct separation of the oxidic and pnictide substructures of different dimensionalities greatly extends the number of adopted structure types through the different altering of common building blocks. This is especially true for the layered oxypnictides, where different stacking of several layers is responsible for structural intricacies. Such a modular approach also helps tuning transport properties through selective stacking of layers or modifying interlayer spacing by selecting interlayer cations.
Among the oxypnictide layers, of specific interest are fluorite-type [M 2 Pn 2] and anti-fluorite-type [AE 2O2] layers, which are formed by fused edge-sharing [MPn 4] or [OAE 4] tetrahedra, respectively (Figure 1i). The presence of the fluorite-type [M 2 Pn 2] layers was shown to be an excellent criterion for the classification of oxypnictides and was implemented in one of the prior reviews on pnictide oxides with d-metals [9]. We acknowledge such approach and partially use it in the current work, yet a range of oxypnictide structures also include structural fragments of different dimensionalities which complicates classification criteria.
3.2 Ternary compounds
Ternary pnictide oxides are usually made of electropositive metallic elements, such as alkaline-earth metals or rare-earth metals coordinated by oxygen and pnictogens. An obvious electronegativity difference between metal cations and non-metal anions results in a noticeable bond polarity and, therefore, charge-balanced compositions, similar to those in Zintl pnictides [13, 14]. A compositional uniqueness of compounds with both O2− and Pn 3− anions often leads to the formation of structure types which are unique for oxypnictides. A list of known ternary pnictide oxides is provided in Table 1.
3.2.1 Ternary “4–2–1” structures with K2NiF4-type
Probably the most abundant and well-studied class of ternary oxypnictide compounds are the “4–2–1” phases with general formula AE 4 Pn 2O (AE = Ca, Sr, Ba, Pn = P, As, Sb, Bi) [19, 20, 33], [34], [35], [36], [37], [38], [39], [40], [41], [42]. Most of the known compositions crystallize in the tetragonal crystal system with the body-centered space group I4/mmm, Pearson symbol tI14. Its structure is isopointal with the anti-perovskite-related K2NiF4 structure (anti-Ruddlesden–Popper) and can be described as corner-sharing oxygen-centered distorted octahedra of AE, forming infinite layers stacked in ABAB-fashion along the c-axis (Figure 2a). Pnictogen coordination resembles a tri-capped trigonal prism polyhedron with nine AE atoms in vertexes (Figure 2b). Several structures, such as Sr4Bi2O, exhibit a noticeable anisotropy of the atomic displacement of the O atoms, indicating the preference for the lower coordination number of 5 instead of 6 for oxygen. Thus, the coordination environment of oxygen can also be viewed as square pyramidal (Figure 2c). Alternatively, the structure can be described as AE-centered octahedrons, where four Pn 3− anions and two O2− anions coordinate one AE site, while another is coordinated by five Pn 3− anions and O2− anion.
(a) Crystal structure of tetragonal AE 4 Pn 2O phase; (b) coordination environment of Pn atoms; (c) square-pyramidal coordination of distorted O atoms in Sr4Bi2O. O atoms are shown as split site with SOF of 0.5; (d) crystal structure of the orthorhombic Ba4P2O phase. AE atoms are dark gray, Pn atoms are orange, and O atoms are red. Unit cells are outlined.
The Ba4P2O phase has similar composition and the chemical bonding can be rationalized in an analogous way, although it crystallizes in a different structure, which resembles anti-La2CuO4 structure type, space group Cmca, Pearson symbol oS28. [OBa6] elongated octahedra are linked via corners to form layers and tilted towards the ab-plane (Figure 2d). All “4–2–1” phases appear to be electron-balanced according to the notation (AE 2+)4(Pn 3−)2(O2−) regardless of the adopted structure type.
In recent years, AE 4 Pn 2O phases attracted much attention beyond structural studies. This family of compounds shows promising ferroelectric and antiferroelectric behavior, while some compounds can polarize even under open-circuit boundary conditions [54]. “4–2–1”-type compounds were predicted to be candidates for photovoltaic materials, although with heavy holes and layered connectivity [55, 56]. Testing for the presence of superconductivity was also performed, yet, was unsuccessful due to the semiconducting nature of these phases [57]. A recent study predicted excellent thermoelectric properties for Ca4Bi2O and Ca4Sb2O semiconductors [32].
3.2.2 Ternary RE m Pn n O m phases
A family of ternary RE m Pn n O m and quaternary (RE,Ca) m Pn n O m derivatives (RE = rare-earth metal; Pn = Sb, Bi) rare-earth-based oxypnictides is constituted by numerous structures with different stacking sequences of rare-earth oxide substructures [43], [44], [45], [46], [47], [48, 58], [59], [60], [61], [62], [63]. A combination of semimetallic binary rare-earth pnictides REPn and insulating rare-earth oxides RE 2O3 was shown to be a potential approach to produce narrow-bandgap semiconducting materials. Ternary and quaternary phases can be synthesized by the direct reactions of rare-earth pnictide, calcium pnictide, or pnictogen with rare-earth oxide in different ratios at relatively high temperatures (Figure 3), which results in a variety of phases with different structural features, bandgaps, and properties.
![Figure 3:
Schematic representation of the synthetic conditions for the selected (RE,Ca)–Pn–O phases. Adapted with permission from [62]. Copyright © 2017 American Chemical Society.](/document/doi/10.1515/zkri-2021-2079/asset/graphic/j_zkri-2021-2079_fig_003.jpg)
Schematic representation of the synthetic conditions for the selected (RE,Ca)–Pn–O phases. Adapted with permission from [62]. Copyright © 2017 American Chemical Society.
These compounds are considered as rare examples of oxygen-rich oxypnictides, and the synthesis, structures, and properties of this family of compounds have recently been reviewed [62]. Here, we will only briefly discuss known subclasses within this large family of compounds. Their crystal structures are very similar and consist of O-centered [ORE 4] edge- and corner-sharing tetrahedra of different arrangements (Figure 4).
![Figure 4:
Structural sequences in the (RE,Ca)
m
Pn
n
O
m
phases. Reprinted with permission from [62]. Copyright ©2017 American Chemical Society.](/document/doi/10.1515/zkri-2021-2079/asset/graphic/j_zkri-2021-2079_fig_004.jpg)
Structural sequences in the (RE,Ca) m Pn n O m phases. Reprinted with permission from [62]. Copyright ©2017 American Chemical Society.
The simplest composition can be described with the formula RE 2 PnO2 (RE = Y, La, Nd Sm, Gd, Ho, Er; Pn = Sb, Bi), which adopt a disordered anti-ThCr2Si2 structure type (space group I4/mmm, Pearson symbol tI10) [43], [44], [45], [46, 59], [60], [61]. The crystal structure can be described as a square net layer made of Pn 2− anions sandwiched between [RE 2O2]2+ anti-fluorite-type layers, which also can be considered as (REO)+ PbO-like slabs (Figure 4). Pnictogens occupy distorted positions, thus making possible Zintl-type [Pn 2––Pn 2–] bonding (Figure 1a), which results in the charge-balanced formula (RE 3+)2(Pn 2−)(O2−)2. A more appropriate crystallographic description of this class of materials was provided for the Pr2SbO2 phase, which has a commensurately modulated orthorhombic crystal structure with the superspace group Immm(0β0)000 [64]. RE 2BiO2 phases undergo an insulator-to-metal transition as early rare-earth elements are replaced by late rare-earth metals [45]. Thermoelectric properties of RE 2SbO2 phases were also reported, yet dimensionless thermoelectric figure-of-merit values do not exceed 0.02 at 400 K [43]. Several “2–1–2” bismuthides were reported to be superconducting with T c values below 3 K [65], [66], [67].
RE 3 PnO3 (RE = La, Sm, Gd, Ho; Pn = Sb, Bi) phases crystallize in their own structure type (space group C2/m, Pearson symbol mS28) [47, 48, 63]. Oxygen-centered corner- and edge-sharing [ORE 4] tetrahedra build up a three-dimensional framework, which hosts Pn atoms inside extended voids formed by two distinct blocks of two and four units (Figure 4). The charge balance is achieved according to the notation (RE 3+)3(Pn 3−)(O2−)3. First-principle calculations and electrical resistivity measurements suggest semiconducting behavior [47, 48]. Higher tetragonal symmetry (space group P42 /mmm, Pearson symbol tP28) is observed for the corresponding quaternary phases with two different rare-earth metals of different atomic radii [63] synthesized at the temperatures of 1550 °C or greater (Figure 3). The structural framework in (RE I RE II)3SbO3 is similar to that of RE 3SbO3, yet, it is formed by units with a different number of edge-sharing [ORE 4] tetrahedra (Figure 4).
RE 8 Pn 3–x O8 (RE = Sm, Gd, Ho; Pn = Sb, Bi) pnictide oxides also adopt novel crystal structure with the space group C2/m, Pearson symbol mS38 [47, 48]. In analogy with RE 3 PnO3, the “8–3–8” crystal structure is formed by REO frameworks built of the O-centered [ORE 4] edge-sharing tetrahedra with empty channels filled with Sb atoms (Figure 4). In addition, an unusual trigonal planar coordination of the oxygen atom was observed in RE 8 Pn 3–x O8 phases. Electronic structure calculations predicted metallic behavior for the idealized RE 8 Pn 3O8 structure, which is one-electron short according to the notation (RE 3+)8(Pn 3−)3(O2−)8(h +). In fact, a deficiency on the Pn site is readily observed for all reported structures, which improves an electron count and results in the experimentally observed semiconducting behavior for RE 8Sb3–x O8 phases [47].
3.2.3 Ternary RE 9 Pn 5O5 phases
RE
9Sb5O5 (RE = La–Pr, Sm, Tb, Dy) were synthesized from Sb2O3, antimony, and the rare earth metals [49], [50], [51]. These compounds represent their own structure type, space group P4/n, Pearson symbol tP38. The crystal structure can be considered as a defect variant of the Sc2Sb structure filled up with oxygen atoms. The structure can also be characterized by 2D fragments of the LaSb structure (NaCl-structure type), where
Crystal structure of the tetragonal AE 9 Pn 5O5 phases (AE denotes rare-earth metals of rare-earth/alkaline-earth mixed occuupied sites). Unit cell is outlined. AE atoms are dark-gray, Pn atoms are orange, and O atoms are red.
3.2.4 Other ternary structures
Ba3Sb2O crystallizes with the space group Pbam, Pearson symbol oP24 with its own structure type [52]. Its anionic substructure can be described with two types of anions – [Sb2]4– dimers and O2− anions separated by Ba2+ counter cations (Figure 6a). The coordination environment of oxygen resembles slightly distorted tetrahedra connected via corners to form chains running along the c-axis. This phase is charge-balanced according to the notation (Ba2+)3[Sb2]4–(O2−) and can be considered as a Zintl phase.
Crystal structures of Ba3Sb2O (a) and Ba3Sb4O (b). Unit cells are outlined. Ba atoms are dark-gray, Sb atoms are orange, and O atoms are red.
Another barium antimonide oxide Ba3Sb4O was synthesized from respective metals and pnictide sesquioxides in alumina crucibles at a temperature as low as 650 °C with the presence of RbSb as additional starting material. It crystallizes in the monoclinic crystal system with the space group P21/c with its own structure [29]. The structural hallmark of this phase are chiral chains
Metallic ternary bismuthide oxide phase Ti8BiO7 was prepared by a high-temperature reaction between Ti and Bi2O3 in inert conditions [53]. It crystallizes in its own structure type (space group Cmmm, Pearson symbol oS32). A noticeable structural feature of the crystal structure is the plethora of different coordinations for O atom, such as square-planar, tetrahedral, and orthogonal pyramidal of Ti (Figure 7). Three Ti sites exhibit three different types of coordination: isosceles-triangle threefold and rectangular fourfold coordination of O atoms and octahedral sixfold coordination of four O and two Bi atoms. Bi atoms form infinite linear chains along the c-axis. With the absence of direct Bi–O bonds, Ti8BiO7 is considered as a heteroanionic one-electron-short system according to the notation (Ti2+)8(Bi−)(O2−)7(h +) [53].
(a) Crystal structure of orthorhombic Ti8BiO7; (b) close-up view of coordination polyhedra and linear Bi chains occured in the Ti8BiO7 structure. Unit cell is outlined. Ti atoms are blue, Bi atoms are orange, and O atoms are red.
3.3 Multinary compounds
Incorporation of additional positively charged species, such as alkali, alkaline-earth, rare-earth or d-metal ions, noticeably expands the number and structural variety of the multinary pnictide oxides compared to the ternary phases [9, 62]. In some instances, quaternary phases are considered as solid-state solutions within the ternary structure types, similar to the previously discussed RE 9Sb5O5 compounds and their Ca-bearing quaternary derivatives [68]. Another example, which unifies ternary and quaternary compounds is the anti-perovskite K4NiF4 structure type, which is observed for “4–2–1” ternary and several quaternary compounds, such as Na2Ti2 Pn 2O (Pn = As, Sb), which are considered as ordered derivatives of “4–2–1” phases [69]. However, the vast majority of the quaternary phases crystallize in their own structure types, although at least 12 of them share identical fluorite-type [M 2 Pn 2] layers (M = transition metal; Pn = pnictogen), responsible for superconducting behavior. Several structure types also feature anti-fluorite [AE 2O2] (AE = alkaline-earth or rare-earth metals) layers typical for ternary phases with anti-ThCr2Si2 structure. The list of known multinary phases is presented in Table 2.
3.3.1 Mixed-cation quaternary compounds
3.3.1.1 Quaternary (RE,Ca) m Pn n O m phases
Iso- or aliovalent substitutions on the cationic sites in ternary oxypnictide phases often result in the formation of quaternary solid solutions with the same structures. The Ca2 RE 7 Pn 5O5 series discussed in the previous section [68] is an excellent example. These phases crystallize with La9Sb5O5-type, typical for the respective ternary compounds [49]. However, incorporating another cation often causes structural perturbations, although crystal structures of ternary and respective quaternary phases remain similar as was previously described for RE 3 PnO3 and (RE I RE II)3 PnO3 compounds.
Several other quaternary derivatives of RE m Pn n O m (RE = rare-earth metal; Pn = pnictogen) ternary oxypnictides offer the formation of structures barely existing for ternary phases. For instance, the family of quaternary oxyantimonides (RE,Ca) m Sb n O m with m = 4 adopt the tetragonal crystal system with the space group I4/m, Pearson symbol tI18 and exist as quaternary solid solutions described by the approximate composition of CaRE 3SbO4 (RE = Ce–Nd, Sm, Gd) [58]. The structural framework can be described as corner-sharing [O2(RE,Ca)6] blocks made by two edge-sharing O-centered tetrahedra (Figure 4). Ca and RE atoms share the same atomic site in ca. 3:1 ratio, thus providing a charge-balanced composition (Ca2+)(RE 3+)3(Sb3−)(O2−)4, which corroborates well with experimentally confirmed semiconducting behavior.
Atomic arrangements in the quaternary Ca2 RE 8Sb3O10 phases (RE = Pr, Nd, Sm, Gd, Tb) are also similar to the previously described ternary and quaternary oxyantimonides, such as CaRE 3SbO4 and RE 3SbO3 (Figure 4), although these quaternary compounds crystallize with a different space group C2/m, Pearson symbol mS46 [58]. Electrical resistivity dependence of temperature for the Ca2 RE 8Sb3O10 compounds also exhibits semiconducting behavior despite the unbalanced electron count for (Ca2+)2(RE 3+)8(Sb3−)3(O2−)10(h +). The origin of such behavior was explained by the localization of Sb p-states near the Fermi level, which arises from the highly disordered Sb layers in the structure.
3.3.1.2 Other transition-metal-free quaternary (A,AE)–Pn–O phases
Admixing monovalent alkali metal (A) and divalent alkaline-earth metal (AE) also enhances structural diversity in pnictide oxides. For instance, two ternary Zintl oxypnictides with formula KBa11
Pn
7O2 (Pn = P, As) were synthesized by the direct reaction between BaPn, Ba, K, and BaO [70]. They crystallize with the space group Fddd, Pearson symbol oF168 with own structure type. The structure contains infinite zigzag chains,
Crystal structures of ABa11 Pn 7O2 (a) and ABa4 Pn 3O (b). Unit cells are outlined. A atoms are pink, mixed-occupied A/AE atoms are black, AE atoms are dark-gray, Pn atoms are orange, and O atoms are red.
Another example of quaternary pnictide oxide phase with two highly electropositive cations is described with the formula ABa4 Pn 3O (A = K, Rb; Pn = Sb, Bi) [29, 71, 72, 116]. These compounds can be synthesized with the mixture of metals and correspondent pnictide sesquioxides in alumina crucibles, similar to the previously described Ba3Sb4O phase. Three known compounds (Table 2) crystallize in the tetragonal crystal system with the space group I4/mcm, Pearson symbol tI36 with Sr5Pb3F-type or stuffed Cr5B3-type, where tetrahedral voids are filled with oxygen. The anionic substructure consists of [Pn 2]4– dimers and isolated Pn 3− and O2− anions. Each atom in the [Pn 2]4– dimer is surrounded by six Ba2+ and two A + cations, building a twice-capped trigonal prism. Coordination polyhedron for the isolated Bi3− anions resembles square antiprism of Ba2+ cations twice capped with K+, whereas O2− cation is tetrahedrally coordinated by four Ba2+ cations (Figure 8b). Two cations are located in different sites and, unlike the previous family, are examples of daltonides, i.e., stoichiometric compounds. ABa4 Pn 3O compositions are charge-balanced Zintl phases according to the notation (A +)(Ba2+)4[Pn 2]4–(Pn 3−)(O2−).
3.3.2 Layered multinary pnictide oxides with fluorite-type [M 2 Pn 2] layers
The schematic view of the fluorite type [M 2 Pn 2] layer is presented in Figure 1i (M = transition metal; Pn = pnictogen). [M 2 Pn 2] layers, as those found in the pnictides with the ThCr2Si2 structure type [117] are considered as one of the common structural blocks for layered pnictides and pnictide oxides with superconducting properties. There are at least 12 known structure types where [M 2 Pn 2] layers are observed. Most of these structures have been comprehensively discussed in the previous reviews [9], [10], [11], [12], however, some structure types are relatively recent and require additional attention.
3.3.2.1 ZrCuSiAs-type
More than 150 quaternary oxypnictides are known for the family of compounds with the general formula REMPnO (RE = lanthanide or actinide; M = d-metal; Pn = pnictogen). “1–1–1–1” phases crystallize with the space group P4/nmm, Pearson symbol tP8, with ZrCuSiAs-type of structure.
A structural description for quaternary compounds with ZrCuSiAs structure can be considered as two different layers, fluorite-type [M 2 Pn 2] and anti-fluorite-type [RE 2O2], both with tetrahedral coordination moieties (Figure 1i). The first layer is formed by edge-sharing [MPn 4] tetrahedra (M = transition metal), where M atoms form square nets with Pn atoms residing above and below. The second layer can be described with oxygen atoms filling tetrahedral voids between electropositive (usually rare-earth) metals (Figure 9a). Interaction between these layers is weak yet noticeable enough to question the distinct layering [6].
![Figure 9:
Most common families of layered pnictide oxides with fluorite-type [M
2
Pn
2] layers (blue) exemplified by the crystal structures of selected representatives. Anti-fluorite-type [AE
2O2] layers are depicted in red. The atom color codes are presented in the formula labels. The unit cells are outlined in black.](/document/doi/10.1515/zkri-2021-2079/asset/graphic/j_zkri-2021-2079_fig_009.jpg)
Most common families of layered pnictide oxides with fluorite-type [M 2 Pn 2] layers (blue) exemplified by the crystal structures of selected representatives. Anti-fluorite-type [AE 2O2] layers are depicted in red. The atom color codes are presented in the formula labels. The unit cells are outlined in black.
Most of the “1–1–1–1” pnictide oxides can be considered as electron-precise Zintl-like phases according to the charge-balanced notation of (RE 3+)(M 2+)(Pn 3−)(O2−) or (RE 4+)(M +)(Pn 3−)(O2−). Such charge-partitioning decently describes compounds with tetravalent cation species such as Th4+Cu+ Pn 3–O2–, yet, calculated charge transfer between cationic [Th2O2]4+ and anionic [Cu2 Pn 2]4– layers were shown to be noticeably smaller compared to such fully ionic approximation [118]. On the other hand, quantitative XPS analysis for Ce-bearing phases, CeMnSbO and CeZnSbO, suggested more complicated charge assignments described as [Ce3.9+O2−][Mn2.8+Sb3−] and [Ce3.3+O2−][Zn2+Sb3−], respectively [119]. Despite the formal charge balance, many “1–1–1–1” phases are multi-band metals with observed superconducting transition, thus they can be barely described as members of the Zintl phases family.
Both direct and flux reactions, among other synthetic approaches, are known to be most successful for the synthesis of REMPnO phases. Metallic Sn or a NaCl/KCl mixture can be used as a flux media, while the source of oxygen usually originates from the loaded oxides, RE 2O3, MO, or Pn 2O3. While the vast majority of the known oxypnictides within this class are thermally stable, we would like to mention two phases, CeZnPO and PrZnPO, which exhibit high-temperature phase transition, with high-temperature modification crystallizing in NdZnPO structure type, vide infra [74].
Tremendous interest in “1–1–1–1” oxypnictide systems emerged after the report of high-temperature superconductivity in the iron-arsenides LaFeAsO1–x F x [120], LaFePO [121], and LaNiPO [122], to name a few. However, this class of compounds is also known for its interesting magnetic behavior, optical and optoelectronic properties [6]. This class also deserves a brief discussion of thermoelectric properties, which were studied for several quaternary and quinary pnictide oxides such as REMPnO [123, 124] and RE 1–x AE x ZnPnO (RE = lanthanides; AE = Ca, Sr; M = Zn, Mn; Pn = As, Sb) [125], [126], [127], [128]. These phases exhibit large thermopower values and low thermal conductivity values, yet high electrical resistivity values limit their applicability for thermoelectric devices.
The vast majority of ZrCuSiAs-based pnictide oxides, their crystal chemistry, magnetic and superconducting properties were comprehensively discussed in previous reviews [6, 8, 129], summarizing the data available for almost all known representatives. Therefore, we prefer to avoid surplus discussion, instead referring the reader to the previously reported works [6, 129, 130], where the complete list of known “1–1–1–1” quaternaries with ZrCuSiAs-type can be found. The current report provides structural parameters for a single example of this phase (Table 2) to introduce typical unit cells for ZrCuSiAs-type structure.
3.3.2.2 RbSm2Fe4As4O2-type
Successful block-layer structural design of the intergrowth structures requires a proper lattice match and charge transfer between distinct block layers, as summarized in the recent report [131]. Excellent examples of such composite materials are ARE 2Fe4As4O2 (A = K, Rb, Cs; RE = Nd, Sm, Gd–Ho) phases, or the so-called “1–2–4–4–2” family, which crystallizes with the space group I4/mmm, Pearson symbol tI26 with the KCa2Fe4As4F2 structure. Its structure is made of alternating blocks of AFe2As2, and 2REFeAsO structures with different types of bonding occurring in isolating [RE 2O2] layers and double asymmetric [Fe2As2] layers (Figure 9) [76, 132, 133]. Due to the lack of precise single-crystal X-ray diffraction or synchrotron data for the “1–2–4–4–2” structure, a single entry on RbSm2Fe4As4O2 in Table 2 exemplifies crystallographic information for this phase.
3.3.2.3 Ba2Cd3–x Bi3O-type
Ba2Cd3–x Bi3O is the only example of its structure and crystallizes in the tetragonal crystal system with the space group I4/mmm, Pearson symbol tI36 [77]. This phase was reported as an unadventurous product in the lead-flux reaction targeting a ternary pnictide phase performed in the alumina crucible. The layered structure is complex and can be viewed as an intergrowth of BaCdBiO slab (ZrCuSiAs-type) and BaCd2Bi2 slab with the CeMg2Si2-structure. PbO-type layers of fused [CdBi4] tetrahedra are alternately stacked along the c-axis with BaO slabs and Bi square-nets (Figure 9). Partial occupancy of one of the Cd sites results in the electron-deficient compositions according to the Zintl-Klemm notation (Ba2+)2(Cd2+)3–x (Bi3−)3(O2−)(h +)2x .
3.3.2.4 Th2Ni3–x P3O-type
Ternary Th2Ni3–x P3O (x ≈ 0.5) phase was first observed as an impurity product in samples aimed at preparing ThNi2P2 phosphide in an alumina crucible, which served as an inadvertent source of oxygen. However, this compound can also be readily synthesized from the mixture of elements and ThO2 [78]. It crystallizes in its own structure type with the space group P4/nmm, Pearson symbol tP18. Two nickel atoms are partially occupied and tetrahedrally coordinated by phosphorus atoms, and both have four nickel neighbors forming a square. These edge-sharing tetrahedra form three fluorite-type [Ni2P2] layers interconnected by slightly elongated P–P bonds (2.34 Å), thus forming a complex [Ni3–x P3] multilayered slab (Figure 9). Oxygen atoms are located in the tetrahedral voids formed by Th atoms, thus forming an oxide anti-fluorite-type [Th2O2] layer, altering a multilayered slab described above. Thus, the structure resembles ZrCuSiAs-type with different stacking sequences. The charge balance in this quaternary phase has been suggested to be (Th4+)2(Ni+)2(Ni0)[P2]5–(P3–)(O2−), where zero-charged Ni was have been considered in the middle of the triple-layer slab. An unusual formal charge of 5– for [P–P] structural fragment was suggested due to the elongated interatomic distance between phosphorus atoms exceeding the sum of covalent radii.
3.3.2.5 U2Cu2As3O-type
U2Cu2As3O was also serendipitously synthesized in a reaction where the initial introduction of oxygen was not intended. The compound came into being from the slight oxidation of the starting materials in the air or from outgassing the silica tubes at high temperatures [79]. The only phase of this structure type crystallizes with the space group P4/nmm, Pearson symbol tP16. It consists of three types of layers, a fluorite-type [Cu2As2], an anti-fluorite-type [U2O2], and a covalently bonded square-planar net of [As4] layers (Figure 9). The latter has been found in several “1–1–2” pnictides [13] yet remains rare for the pnictide oxides. An electron charge-balance can be broken down to the (U4+)2(Cu+)2(As3−)2(As2−)(O2−) notation. While U2Cu2As3O remains only the oxypnictide with this structure, a prior report on U4Cu4P7 was later revised in favor of U2Cu2P3O composition, which was believed to be isostructural to the described quaternary arsenide phase [79].
3.3.2.6 Sm2CuAs3O-type
Crystals of Sm2CuAs3O were obtained as a by-product of the alkali-halide flux synthesis targeted in the SmCuAs2 phase [80]. It was believed the oxygen originated from silica tubes, whereas attempts to synthesize this phase from different oxides were not successful. Sm2CuAs3O crystallizes in the orthorhombic crystal system with the space group Pnma, Pearson symbol oP28 and exemplifies the only pnictide oxide phase of distorted Hf2CuGe4 structure type. Its layered crystal structure contains fluorite-type [Cu2As2] and anti-fluorite-type [Sm2O2] layers that were described previously. However, these layers are separated by planar sheets of Sm and distorted square nets of As atoms, which have to be preferably described as planar infinite twinned zigzag chains [As−] n running along the b-axis (Figure 9). Such chains are the hallmark of this structure and are unique for the quaternary oxypnictides with d metals. The Zintl–Klemm concept is also applicable for the discussed compound, which is believed to be electron-balanced according to the notation (Sm3+)2(Cu+)(As−)2(As3−)(O2−). While semiconducting behavior for this compound is expected, a structural similarity with known superconductive phases hints at possible superconductivity transition.
3.3.2.7 La3Cu4P4O2-type
Unlike previously described layered pnictide oxides, where one member represented one structure type, a small family of lanthanide transition metal pnictide oxides RE 3 M 4 Pn 4O2 (RE = La–Nd, Sm; M = Ni, Cu Pn = P, As) contain several compounds which crystallize with the space group I4/mmm, Pearson symbol tI26 [81], [82], [83], [84], [85, 134]. Compounds adopt a layered crystal structure with own structure, which resembles an ordered Zr3Cu4Si6-type. The structure is formed by anti-fluorite type [RE 2O2] (B) layers made of condensed edge-sharing [ORE 4/4] tetrahedra stacked with fluorite-type [M 2 Pn 2] (A) layers tetrahedra in the sequence of –B–B–A– (Figure 9). Two [M 2 Pn 2] layers are linked by Pn–Pn bonds, similar to the previously described Th2Ni3–x P3O phase. The presence of two different [Pn 2]4– and Pn 3− units was determined by the 31P solid-state NMR method on P-bearing phases. Precise structural refinement showed partial occupancy for the oxygen position resulting in ca. 1.5 oxygen atoms per formula unit. Therefore, chemical bonding can be rationalized with the charge-balance formulation (RE 3+)3(Cu+)4[Pn 2]4–(Pn 3−)2(O2−)1.5 and is expected to behave as semiconductors. However, “3–4–4–2” phases with Ni or Cu phases exhibit metallic behavior with rare examples of superconducting transition (T c = 2.2 K) as was reported for La3Ni4P4O2 compound [81, 134, 135].
La5Cu4 Pn 4O4Cl2 (Pn = P, As) can be considered a structural derivative of the “3–4–4–2” phase. These complex mixed-anion quinary compounds can be readily prepared from a cold-pressed pellet of La, Cu2O, P, LaCl3 (phosphide), and La, As, Cu2O, LaOCl (arsenide) from NaCl flux [83, 85]. It adopts a new structure type with space group I4/mmm, Pearson symbol tI38. It can be described as an intergrowth of LaOCl slabs, ZrCuSiAs- and La3Cu4P4O2-related slabs (vide supra) (Figure 9). Structural relationships between these slabs can be expressed as 2LaOCl + La3Cu4P4O2 = La5Cu4P4O4Cl2, which makes La5Cu4As4O4Cl2 a strongly anisotropic hybrid material composed of covalently bonded metallic [CuAs4/4] layers located within ionic insulating oxide and chloride blocks. This phase is one-electron short according to the fully ionic partitioning notation (La3+)5(Cu+)4(As3−)2[As4]4–(O2−)4(Cl−)2(h +).
3.3.2.8 Sr2Mn3As2O2-type
Several layered quaternary oxypnictides AE 2 M 3 Pn 2O2 (AE = Sr, Ba; M = Cr, Mn, Zn; Pn = P, As, Sb, Bi) were obtained by the high-temperature reaction between correspondent amounts of metals and/or metal oxides. They crystallize in their own structure type, space group I4/mmm, Pearson symbol tI18, and are considered as two-dimensional magnetic materials with high magnetic ordering temperatures [7, 9, 86], [87], [88, 136], [137], [138], [139]. The crystal structure of the AE 2 M 3 Pn 2O2 family of compounds can be described as a 1:1 intergrowth of fluorite-type [M 2 Pn 2] layers, similar to those in BaAl4-type with tetrahedral coordination of M, and [M’O2] layers with square-planar coordination of metal, which occur in cuprate superconductors in the form of [CuO2] planes (Figure 9). Sr2+ and Ba2+ cations serve as countercharges and are located between anionic layers. “2–3–2–2” phases are formally charge-balanced according to the notation (AE 2+)2(Mn2+)3(Pn 3−)2(O2−)2 suggesting semiconducting behavior.
Ordered quinary variants of Sr2Mn3As2O2 structure were reported as well. A clear layer separation results in the formation of phases, where different 3d-metals, such as Cr, Fe, Cu, or Zn, substitute Mn atoms predominantly in [Mn2As2]2– layers. Resulting quinary compounds with formulae Sr2Mn2.23Cr0.77As2O2 [93], Sr2CrFe2As2O2 [140], Sr2Mn2CuAs2O2 [92], AE 2MnZn2As2O2 [90,91], and Sr2CrCo2As2O2 [141] are made up from square planar [MnO2]2– or [CrO2]2– layers with layers of [M 2As2]2– tetrahedra interspersed by alkaline-earth cations.
3.3.2.9 Sr2ScFeAsO3-type
The family of quinary AE 2 M′M′′PnO3 (AE = Ca, Sr, Ba; M′ = Al, Sc, Ti, V, Cr; M′′ = Fe, Cr; Co; Pn = P, As) superconductive phases crystallizes in the tetragonal crystal system with the space group P4/nmm, Pearson symbol tP16 with Sr2CuGaSO3 structure [94, 95, 142], [143], [144], [145], [146], [147], [148]. These layered pnictide oxides can be readily synthesized by the solid-state reaction between metals, metal oxides, and/or binary metal pnictides. The crystal structure can be described as alternatively stacking fluorite-type [Fe2 Pn 2] layers and perovskite-type AE 4 M 2O6 (=2AE 2 MO3) layers as presented in sky-blue in Figure 9. Five O atoms coordinate M′ atom in a square-pyramidal fashion. Therefore, their composition can also be described as (AE 4 M′2O6)(M′′2 Pn 2), emphasizing the presence of constituting layers. Sr2ScFePO3 and Sr2VFeAsO3 undergo superconducting transition at T c ≈ 17 and 37 K respectively under ambient pressure [142, 149], whereas V-bearing samples exhibit the relatively high value of T c ≈ 46 K under high pressure [150].
3.3.2.10 Sr3Sc2Fe2As2O5-type
AE 3Sc2 M 2As2O5 (AE = Sr, Ba, Eu; M = Cr, Fe, Co, Ni) layered oxide arsenides were also synthesized by high-temperature solid-state method from the mixture of metals, metal oxides and/or metal arsenides [96, 97, 144, 151], [152], [153], [154], [155]. These phases crystallize with space group I4/mmm, Pearson symbol tI28, with Sr3Fe2O5Cu2S2-type structure. Similar to the previously described pnictide oxide phases, the “3–2–2–2–5” family of compounds can be described as a layered structure where fluorite-type [M 2As2] layers stack alternately with the perovskite oxide layer (ScO2)(AEO)(ScO2) (Figure 9). Two AE atoms exhibit 12-fold (to 12 oxide anions) and 8-fold coordination (to four oxide and four arsenide anions). Scandium is surrounded by five oxide anions in square-pyramidal coordination. A homologous series was observed for the “3–2–2–2–5” family, which can be described as fluorite-type [Fe2As2] layers separated by double or triple perovskite layers. Resulting compositions can be generally described as (M 2As2)[AE n+1 M′ n O y ] or (M 2As2)[AE n+2 M n O y ] (AE = Ca, Sr, Ba; M′ = Mg, Al, Sc, Ti; n = 3–5) with reported compositions of Ba4Sc2Ni2As2O6 (similar to the Sr2ScFeAsO3-type), Sr4(Sc,Ti)3Fe2As2O8, Ba4Sc3Ni2As2O8, Ca5(Sc,Ti)4Fe2As2O11, and Ca6(Sc,Ti)5Fe2As2O14 to name a few [154], [155], [156], [157], [158], [159].
Sr- and Ba-bearing samples are not superconductive and show no structural anomaly or magnetic ordering [151]. Rare examples of superconductive compounds within this family have composition Ca3Al2Fe2 Pn 2O5–y (Pn = P, As) with a transition temperature T c of ca. 30 K for arsenide and 16.6 K for phosphide [153]. However, the correspondent phosphide-arsenide solid solution shows no evidence of superconductive behavior [160]. Compounds in (Fe2As2)[Ca n+1 M n O y ] series are also superconductive with relatively high T c values of up to 42 K [156].
3.3.3 Layered multinary pnictide oxides without fluorite-type [M 2 Pn 2] layers
3.3.3.1 NdZnPO-type
As was previously mentioned, several “1–1–1–1” quaternary phases, such as CeZnPO and PrZnPO with ZrCuSiAs-type, undergo high-temperature phase transition with the formation of rhombohedral modification with NdZnPO structure, space group
Crystal structure of layered rhombohedral NdZnPO structure. Unit cell is outlined. Nd atoms are dark-gray, Zn atoms are blue, P atoms are orange, and O atoms are red.
3.3.3.2 Na2Ti2As2O-type
Na2Ti2
Pn
2O phases (Pn = As, Sb) crystallize in their own structure type with the space group I4/mmm, Pearson symbol tI14, which can also be considered ordered anti-K2NiF4-type [98]. The successful synthesis should start with the Na2O/Na2O2 mixture, which serves as the source of oxygen [69]. An anionic substructure of this family of compounds consists of edge-sharing
Crystal structure of Na2Ti2As2O (top) and BaTi2As2O (bottom). Unit cells are outlined. Na atoms are pink, Ba atoms are dark-gray, Ti atoms are blue, As atoms are orange, and O atoms are red.
The temperature dependent electrical resistivity for these compounds exhibits an anomaly reminiscent of charge-density-wave/spin-density-wave materials. An electron balance in Na2Ti2 Pn 2O quaternary phases can be described according to the notation (Na+)2(Ti3+)2(Pn 3−)2(O2−). The As-bearing compound shows an insulator-to-insulator transition around 135 K, whereas the Sb-bearing phase shows a metal-to-metal transition around 120 K.
Yb4Sc4Sb4O is a single representative in the correspondent compositional diagram, and it was synthesized in an attempt to produce the ScYbSb ternary compound [99]. However, the latter La2Sb-type phase exists if stabilized by oxygen atom similar to the previously described RE 4 Pn 2O phases and Na2Ti2 Pn 2O phase, with the difference of ca. 0.5 site occupation factor assigned for the oxygen atom. Structures of Yb4Sc4Sb4O and Na2Ti2 Pn 2O are identical and, therefore, the former will not be discussed in depth.
3.3.3.3 BaTi2As2O-type pnictide oxides and derivatives
BaTi2 Pn 2O (Pn = As, Sb, Bi) phases are also structurally related to the Na2Ti2 Pn 2O compounds with the difference of single layer of Ba atoms along the c-axis (Figure 11). Compounds crystallize in the tetragonal crystal system with the space group P4/nmm, Pearson symbol tP6 with own structure type [100], [101], [102]. The resistivity studies for the As-bearing exhibit anomaly at 200 K, which was suggested to be a spin-density wave or phase transition [100]. Suppressing this anomaly by doping carriers often results in emerging of superconductivity. Indeed, the isostructural Sb-bearing phase exhibits superconducting transition at T c = 1.2 K [101], whereas critical temperature T c for its Na- and K-doped quaternary phases increases to 5.5 and 6.1 K, respectively [161, 162]. Heavier Bi-bearing analog indicates an increased T c of 4.6 K [102]. A recent study reported pressure-induced transformation of BaTi2 Pn 2O structure associated with unusual Pn–Pn bond elongation (Pn = As, Sb) [163], yet such structural rearrangement results in the decreased T c values for Na-doped BaTi2Sb2O solid solutions [164]. However, T c for the superconducting sample of BaTi2Bi2O monotonically increased with the pressure up to 4.0 GPa followed by saturation [165]. More insights on superconductivity in the titanium pnictide oxides can be found in the recent review [166].
The superconductive nature of BaTi2 Pn 2O phases stimulated synthetic efforts to synthesize more complex layered oxypnictides based on [Ti2 Pn 2O]2– slabs. Beyond bare alkali and alkali-earth metal cations, complex units, such as [(SrF2)2]2+ or [(SmO)2]2+ serve as interlayered cations [167]. In addition, the previously discussed modular approach can also be applied for the superconductive systems based on [Ti2 Pn 2O]2–. For instance, the BaTi2As2O phase can be modified to synthesize quinary compounds with the formula Ba2Ti2 M 2As4O (M = Fe, Cr), which can be viewed as an intergrowth of layered BaM 2As2 and BaTi2As2O phases, thus containing [M 2As2] layers and [Ti2O] sheets (Figure 9) [103, 104]. Such “2–2–2–4–1” phases crystallize with space group I4/mmm, Pearson symbol tI22 with an expected c-axis of c “2–2–2–4–1” = c “1–2–2” + 2c “1–2–2–1”. While these phases contain the fluorite-type [M 2As2] layer, they also belong to the group of compounds discussed in Section 3.3.2. However, they are direct structural derivatives of the BaTi2 Pn 2O phases, therefore presented herein.
3.3.3.4 Ba2Mn2As2O-type and Ba2Mn2Sb2O-type
Ba2Mn2 Pn 2O (Pn = As, Sb, Bi) oxypnictides were synthesized at high temperatures from the mixture of metals and Pn 2O3 or MnO2 powder as the source of oxygen [105, 106, 168]. Members of this family adopt two different structure types, with arsenide oxide member crystallizing in its own structure type, monoclinic crystal system, space group C2/m, Pearson symbol mS14, whereas heavier oxypnictides adopt hexagonal crystal system, space group P63/mmc, Pearson symbol hP14 with own structure. Both structures share similar features and are formed by the same type of distorted [MnSb3O] corner-sharing tetrahedra linked together by O atoms, thus forming linear Mn–O–Mn fragment, whereas edge-sharing connection is realized through the Sb/Bi atoms (Figure 12). However, layered anionic substructures are slightly different for both types of compounds. Ba2Mn2As2O phase is single-layered, composed of infinite, corner-sharing [Mn2O] and edge-sharing [Mn2Sb2] connections (Figure 12a), whereas heavier oxypnictides form anionic double layers [Mn2Sb2O]2– oriented parallel to the ab-plane (Figure 12b). While O atoms bridge sublayers, a prolongation is achieved via edge-sharing [Mn2 Pn 2] connections.
Crystal structures of monoclinic Ba2Mn2As2O (a) and hexagonal Ba2Mn2Sb2O (b). Unit cells are outlined. Ba atoms are dark-gray, Mn atoms are blue, Pn atoms are orange, and O atoms are red.
Physical properties were studied for the Sb-bearing compound. Magnetic susceptibility and specific heat show anomalies at T N = 60 K, consistent with antiferromagnetic ordering. Charge balanced notation (Ba2+)2(Mn2+)2(Sb3−)2(O2−) suggest a semiconducting behavior, which corroborates well with both calculations and experiment reporting energy gaps of ca. 0.686 and 0.59 eV, respectively [168].
3.3.3.5 Ba5Cd2Sb4O2-type
A quaternary oxyantimonide, Ba5Cd2Sb4O2, was also synthesized as a side product in the reaction targeted in the ternary pnictide phase [107]. It crystallizes in the monoclinic crystal system with the space group C2/m, Pearson symbol mS26. The anionic substructure consists of polyanionic
Crystal structure of monoclinic Ba5Cd2Sb4O2 phase. Unit cell is outlined. Ba atoms are dark-gray, Cd atoms are blue, Sb atoms are orange, and O atoms are red.
3.3.3.6 Nd10Au3As8O10-type
Four transition metal pnictide oxides RE 10 M 3As8O10 (RE = Nd, Sm; M = Au, Pd) were synthesized from the mixture of rare earth elements, appropriate rare-earth sesquioxides, arsenic, arsenic oxides, and gold/palladium [108, 109]. They crystallize with the space group I4/m with Nd10Au3As8O10 structure type, Pearson symbol tI62. Two different layers can be distinguished for this layered crystal structure (Figure 14). Oxide anions are tetrahedrally coordinated by rare-earth metals, thus forming positively charged [Nd10O10]10+ layers made of edge-sharing tetrahedra. The second planar layer is formed by arsenic and gold atoms, where the latter are bonded to four arsenic dumbbells in square-planar coordination or distorted rectangular coordination (Figure 1e). Such layers have been already observed in pnictide compounds with the formula Ca10(MAs)10(Pt3As8) (M = transition metal) [170]. While this type of coordination is common for Pd2+ atoms, such an environment is rare for Au. Nd10Pd3As8O exhibits a structural phase transition near 160 K according to the resistivity data. Low-temperature modification crystallizes in monoclinic crystal system with the space group C2/c (Table 2), which results from periodic displacements of the palladium atoms out of the mirror plane.
Crystal structure of Nd10Au3As8O10 phase. Unit cell is outlined. Nd atoms are dark-gray, Au atoms are blue, As atoms are orange, and O atoms are red.
Pd-bearing compounds are charge-balanced according to the notation (Nd3+)10(Pd2+)3(As2−)8(O2−)10 and exhibit experimentally confirmed semiconducting behavior [109]. The electron-precise description is somewhat schematic due to the comparable electronegativity values of Pd and As, yet calculations confirm a covalent character of the layer. At the same time, resistivity measurements for isotypic gold compounds display a metallic character of studied compounds [108].
3.3.4 Non-layered multinary pnictide oxides
3.3.4.1 AE5M2Pn5O-type
Several quaternary phases with the formula AE 5 M 2 Pn 5O x (AE = Ba, Eu; M = Zn, Cd; Pn = As, Sb; x = 0.5–1) crystallize with the centrosymmetric space group Cmcm, Pearson symbol oS52), with Eu5Cd2Sb5F structure [110], [111], [112]. An anionic substructure can be described as one-dimensional double-stands of corner-shared MPn 4 tetrahedra with Pn–Pn bonds that connect the tetrahedra [M 2 Pn 5]9– to form pentagonal quasi-channels with one out of five AE atoms is contained within the channels. An isolated O2− anion is tetrahedrally coordinated by four AE 2+ cations (Figure 15a). Although the initial discovery of this structural family was serendipitous, it was possible to achieve quantitative yields from reactions between elements and barium peroxide in the case of Ba5Cd2Sb5O0.5. An approximate 50%-occupancy of oxygen satisfies charge-balance conditions according to the notation (Ba2+)5(Cd2+)2(Sb2−)2(Sb3−)3(O2−)0.5 and confirmed by electronic band structure calculations [111]. At the same time, Eu-bearing phases are Eu5 M 2 Pn 5O (M = Zn, Cd; Pn = As, Sb) are one-electron-short narrow bandgap semiconductors according to the notation (Eu2+)5(M 2+)2(As2−)2(As3−)3(O2−)(h +), which hints at the possibility of mixed-valent Eu2+/Eu3+ state [110, 112].
Crystal structures AE 5 M 2 Pn 5O (a), RE 2AuP2O (b), and RE 9Au5–x Pn 8O6 (c). Unit cells are outlined. AE and RE atoms are dark-gray, M and Au atoms are blue, Pn atoms are orange, and O atoms are red.
3.3.4.2 La2AuP2O-type
RE 2AuP2O (RE = La, Ce, Pr, Nd) phosphide oxides were obtained from mixtures of the rare-earth elements, binary rare-earth oxides, gold powder, and red phosphorus in sealed silica tubes [113, 114]. Compounds crystallize in a monoclinic crystal system with the space group C2/m, Pearson symbol mS24 with La2AuP2O structure type. The anionic substructure consists of [AuP2]4– polyanions in a hexagonal rod packing motif. Such substructure is characterized by reduced dimensionality, where gold atoms exhibit distorted trigonal-planar phosphorus coordination of P3– anions and [P2]4– dumbbells (Figure 15b). Cationic layers can be described as cis-edge-sharing [ORE 2/2 RE 3/3] tetrahedra located between polyanionic ribbons. RE 2AuP2O phases are charge-balanced according to the notation presented to the doubled composition (RE 3+)4(Au+)2[P2]4–(P3–)2(O2−)2.
3.3.4.3 Ce9Au5As8O6-type
Several RE–Au–As–O ternary system members can be described with the formula RE 9Au5–x As8O6 (RE = Ce, Pr; x ≈ 0.09–0.25) [115]. They have been synthesized similarly to the previously described RE 10 M 3As8O10 phases with the incorporation of the rare-earth oxides into the reaction mixture. These compounds represent a new structure type and crystallize with space group Pnnm, Pearson symbol oP116. An anionic substructure consists of a complex 3D network made by [Au5–x As8](15+x)– ions linked by homoatomic As–As dumbbells as illustrated in Figure 15c. Two gold sites exhibit positional disorder and are partially occupied, which results in departing from the charge balance. Six crystallographically independent oxygen atoms with distorted tetrahedral RE coordination are condensed in the edge-sharing linkage, which resembles a stripe oriented along the c-axis. Thus, [ORE 4/4] and [ORE 4/3] tetrahedra form [RE 4O3]2 12+ cationic unit, which along with RE 3+ cations maintain charge balance described by an electron precise formula (Pr3+)9(Au+)5(As3−)4([As2]4–)(O2−)6 [115].
4 Summary and outlook
Pnictide oxides belong to the unique class of heteroanionic compounds composed of oxide (O2−) and pnictide (Pn 3−) anions. Several hundreds of compounds formed by more than 40 elements were gathered within nine ternaries and at least 28 multinary structure types. Layered d-metal oxypnictides dominate compositional and structural abundancy with more than 200 compositions of 20 different structures. However, the field of pnictide oxides with 4d-, 5d-elements, or group 13 metals, is still relatively unexplored. A newfangled report on gallate pnictides Ba9[GaO4]3 Pn (Pn = Sb, Bi) and Zintl phase Ba2[GaO2As) indicates the conceptual existence of such class of materials [171].
Further developments of novel inorganic pnictide oxides are eagerly anticipated as research on such materials is aligned with the current trends in materials chemistry and physics. Exploration and study of compounds with multiple anions has attracted and will continue to attract much attention [3]. Due to the difference of radii, charge, and electronegativity for pnictide and oxide anions, dimensionality, crystal and electronic structures, and physical properties can be controlled. The presence of two different anions increases the tunability of the bandgap, thus can be beneficial for the band engineering and further tuning structure-properties relationships. The applicability of pnictide oxide systems is underestimated, focusing primarily on the superconducting properties of layered d-metal pnictide oxides. However, the enormous structural diversity of semiconducting oxypnictides may promote different applications, such as thermoelectric, non-linear optical, or luminescent.
While explorational synthesis remain a primary approach for the discovering of novel oxypnictides, a high-throughput computational methods already show a promise in predicting hitherto unknown pnictide oxides [172]. We also point out the necessity of revising the properties of known structures, which will enhance the progress on the rapidly developing field of heteroanionic materials.
Funding source: US Department of Energy
Award Identifier / Grant number: DE-SC0008885
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Author contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.
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Research funding: This minireview is based upon extensive prior research published in over 170 publications. Of those, some are authors’ own work, which has received financial support from the US Department of Energy through a grant DE-SC0008885.
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Conflict of interest statement: The authors declare no conflicts of interest regarding this article.
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Articles in the same Issue
- Frontmatter
- In this issue
- Micro Review
- Structural diversity among multinary pnictide oxides: a minireview focused on semiconducting and superconducting heteroanionic materials
- Inorganic Crystal Structures (Original Paper)
- The orthorhombic-to-monoclinic phase transition in NbCrP – Peierls distortion of the chromium chain
- Halide-sodalites: thermal expansion, decomposition and the Lindemann criterion
- RE3Rh2Sn4 (RE = Y, Gd–Tm, Lu) – first stannides with Lu3Co2In4 type structure
- Scandium–copper–indides deriving from the ZrNiAl and MnCu2Al type structures
- Syntheses, crystallographic characterization, and structural relations of Rb[SCN]
- Organic and Metalorganic Crystal Structures (Original Paper)
- Unique supramolecular assembly of a synthetic achiral α, γ-hybrid tripeptide
Articles in the same Issue
- Frontmatter
- In this issue
- Micro Review
- Structural diversity among multinary pnictide oxides: a minireview focused on semiconducting and superconducting heteroanionic materials
- Inorganic Crystal Structures (Original Paper)
- The orthorhombic-to-monoclinic phase transition in NbCrP – Peierls distortion of the chromium chain
- Halide-sodalites: thermal expansion, decomposition and the Lindemann criterion
- RE3Rh2Sn4 (RE = Y, Gd–Tm, Lu) – first stannides with Lu3Co2In4 type structure
- Scandium–copper–indides deriving from the ZrNiAl and MnCu2Al type structures
- Syntheses, crystallographic characterization, and structural relations of Rb[SCN]
- Organic and Metalorganic Crystal Structures (Original Paper)
- Unique supramolecular assembly of a synthetic achiral α, γ-hybrid tripeptide
