Synthesis and structure determinantion of the first lead arsenide phosphide Pb₂As_xP_14–x (x ~ 3.7)

2.1 Synthesis

Starting materials for the synthesis of polycrystalline samples of Pb₂As_xP_14–x were powders of lead (abcr, Karlsruhe, Germany), arsenic (abcr) and pieces of phosphorus (ChemPUR, Karlsruhe, Germany), all with stated purities better than 99%. The elements were mixed in a ratio of Pb to (P, As) of 1:1 in a mortar, subsequently cold-pressed in to pellets of 6 mm diameter and sealed in evacuated quartz tubes. The ratio of P to As was varied between 3:7 and 5:5. Heating of the starting materials took place in a muffle furnace. We applied three ramps each one followed by a holding step: Ramp 1 to 423 K within 48 h, holding period at that temperature for 48 h; Ramp 2: 573 K within 48 h and 48 h holding; Ramp 3: 673 K within 48 h and 48 h holding period. Finally, the samples were cooled to room temperature over a period of 48 h. The lead flux was dissolved in a 1:1 mixture of H₂O₂ (35%, ACROS, Fisher Scientific, Nidderau, Germany) and glacial acetic acid (96%, VWR International, Darmstadt, Germany) in an ultrasonic bath. Pb₂As_xP_14–x is slightly less soluble in the H₂O₂-glacial acetic acid mixture than the lead matrix. Therefore, the maximum treatment of the crude product should not exceed several min. Already after 10 min the crude product is dissolved completely. Due to this behavior, lead and arsenic always remained as side products in the polycrystalline sample and single crystals were of low quality. A representative crystal picture is given in Fig 1. The resulting powder is black and stable in air for several weeks.

Fig. 1:

Crystal structure and SEM picture of Pb₂As_xP_14–x. (a) View along the crystallographic a axis. (b) Polypnictide substructure composed by rows of condensed (Pn)₆ rings connected by cis- and trans-oriented P bridges. (c) Coordination of Pb1 and Pb2 including bond lengths (in Å) and orientation of lone pairs.

2.2 X-ray diffraction

Powder XRD measurements of the remaining Pb₂As_xP_14–x products were carried out on a STOE Stadi P diffractometer (CuK_α1 radiation, λ = 1.54051 Å; germanium monochromator) in transmission using a flatbed sample holder. Analysis of the data was carried out with the program package WinXpow [13]. The resulting diffractograms showed reflections of the title compound together with those of non-dissolved lead and arsenic. Lattice parameters of isolated polycrystalline materials from different batches (molar ratio of P to As of 3:7 and 5:5) ranged from a = 10.078(4) to 10.092(3), b = 10.484(3) to 10.495(2) and c = 13.666 to 13.671 Å, pointing towards a certain but not pronounced phase width.

Several crystals of Pb₂As_xP_14–x were glued to small quartz fibers using bees wax. Single-crystal intensities data were collected at room temperature using a STOE IPDS-II diffractometer (MoK_α radiation; λ = 0.71073 Å; graphite monochromator). Numerical absorption corrections were applied to the data and a structure refinement was performed for each experiment. The collected data sets were corrected for polarization and absorption effects by the aid of the XShape [14] and Xred [15] routines. The structure solution and refinement (full-matrix least-squares on F²) has been performed with the Superflip routine [16], implemented in the program Jana 2006 [17].

2.3 SEM and EDX data

Semi-quantitative EDX analyses and pictures of crystals remaining after the hydrogenperoxide/glacial acid treatment were carried out by using a JEOL JCM-6000 scanning electron microscope fitted with a JED 2200 detector in high vacuum mode. We also determined the composition of crystals directly mounted on the fibers used for single crystal structure determination. All measurements substantiated a certain phase width for a given starting composition in the solid solutions Pb₂As_xP_14–x. The maximum arsenic content derived from our analyses was x = 5.8(2) independent of the batch. We could not detect any impurity elements heavier than sodium in our samples. Obviously, the supplied arsenic was not fully incorporated in the structure of Pb₂As_xP_14–x. For the Pb₂As_3.7(1)P_10.3(1) crystal used for structure determination we found Pb: As: P = 16(2): 14(2): 69(4) in at% (refined composition in at%: 12.5: 23.1(7): 64.4(6)). We need to point out, that the accuracy of the EDX analysis is lower for the direct measurement of the mounted single crystals than for a common metallographic analysis.

3.1 Structure refinements

Single crystals have been isolated from two different batches (molar ratio of P to As of 3:7 and 5:5) and a structure determination was performed for each one. Due to the synthesis conditions stated above, the crystal quality varied drastically, and we found crystals with varying P to As ratio in the range of x = 3.7–4.8, according the sum formula Pb₂As_xP_14–x. In the following, we report only on the best candidate Pb₂As_3.7(1)P_10.3(1). All relevant crystallographic data of Pb₂As_3.7(1)P_10.3(1) are given in Tables 1–3. In order to verify and substantiate the refined composition, all crystals were subject to quantitative EDX analysis after the single crystal structure determination. Pictures of two crystals representing the minimal and maximal arsenic content (mounted on glass capillaries) are given as Supporting Information available online.

The data sets revealed primitive orthorhombic lattices and the only systematic extinctions h00 with h = 2n, 0k0 with k = 2n and 00l with l = 2n led to the space group P2₁2₁2₁ (No. 19). The calculated Flack parameter was close to 0.5 and the structure was subsequently refined as an inversion twin. Twin volumes are 0.51(4) and 0.49. Platon [18] was used to check for higher metric symmetry and no other space group than the given one was suggested. A check of precession plots did not show any hints for a symmetry reduction We observed 16 electron density maxima, which we assigned to two lead and 14 pnictide sites due to bond length considerations and differences in electron density. A separate refinement of the occupancy parameters led to mixed P/As occupancies for most pnictide sites. The site occupancy factors for all pnictide sites were restricted to 1. Due to the low crystal quality, an extinction correction was not necessary. The refined atomic positions, displacement parameters, and interatomic distances of Pb₂As_3.7(1)P_10.3(1) are given in Tables 1–4.

Further details of the crystal structure investigation may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail: crysdata@fiz-karlsruhe.de, http://www.fiz-karlsruhe.de/request_for_deposited_data.html) on quoting the deposition number CSD-431033 (Pb₂As_3.7(1)P_10.3(1)).

3.2 Crystal chemistry

Recently, the discovery and examination of two-dimensional monolayer materials of heavy group 15 elements lead to huge interest in the development and optimization of elemental pnicogen based and related materials [19]. Related to graphene, mono-layer and multi-layer materials of phosphorus, arsenic and antimony have been either prepared or predicted theoretically [20–24]. Heavy doping of such materials and the search for binary and ternary derivatives is of interest due to the expected improvement of physical properties like charge carrier mobility, thermoelectric behavior or optoelectronic properties [12, 20, 22]. Examples of black phosphorus-related binary materials are BaP₃ [25, 26], SnP₃ [27] or PbP₇ [3, 4]. They adopt substructures, which are structurally correlated with black phosphorus and gray arsenic by a replacement of selected atoms in the element.

The discovery of PbP₇ [3, 4] marks a milestone in solid state phosphorus chemistry because the formation of a binary lead phosphide had been tried for decades without success. PbP₇ and black phosphorus are structurally related in such a way that the polyphosphide substructure of PbP₇ can be regarded as a partially substituted and strongly corrugated derivative of black phosphorus and gray arsenic. Formally, Pb²⁺ replaces some P atoms of a black phosphorus monolayer and coordinates either the resulting 2-bonded P atoms or lone pairs of neighboring P atoms pointing towards their direction.

Starting from PbP₇ we intended to replace P by As following Vegard’s rule for isotypic structures. As stated in the Introduction, such a Vegard-like behavior was observed for the solid solution related to black phosphorus and black arsenic [12]. Surprisingly, neither an isotypic structure nor a Vegard-like behavior was observed. In the following we discuss similarities and differences of the two lead pnictides Pb₂As_3.7(1)P_10.3(1) and PbP₇.

The structure of Pb₂As_xP_14–x (exemplarily discussed for Pb₂As_3.7(1)P_10.3(1)) contains 16 crystallographically independent sites, two lead and 14 pnictide (Pn) ones. All Pn sites are mixed-occupied by P and As in various ratios (Fig 1b). Pb₂As_3.7(1)P_10.3(1) consists of strongly corrugated Pn layers located parallel to [110]. In Fig 1, a section of the crystal structure, the polypnictide substructure and the coordination of the two crystallographically independent lead positions are given.

Within these polypnictide layers, ribbons of condensed (Pn)₆ rings in chair conformation (Fig 1b) are running along the a axis. Pn bridges or Pb²⁺ cations (Pb1) connect these ribbons to neighboring ribbons forming a strongly corrugated polypnictide net. Only Pb2 is coordinating this net in the third dimension.

In Pb₂As_3.7(1)P_10.3(1), the bond distances of 2.233–2.384 Å within the (Pn)₆ rings are longer than an average P–P single bond in black phosphorus (2.228 Å [1]), which can be explained by the greater atomic radius of arsenic. Two out of fourteen Pn sites are fully filled by phosphorus, each coordinated to Pb1 and Pb2. Obviously, lead tends to be preferably bonded to P instead of As. All other Pn sites are mixed occupied with variable amounts of P and As. Pn3 to Pn14 are forming the (P/As)₆ ring strands. Pn1, Pn2 and Pb1 in a ratio of (Pn1+Pn2) to Pb of 2:1 are at the bridging positions to neighboring (Pn)₆ ring strands in 1,4 position. The stereochemistry of the bridging atoms determines the dimensionality of the whole structure. In Fig 2 the stereochemistry of the (Pn)₆ ring fragments for PbP₇ [3] and Pb₂As_3.7(1)P_10.3(1) is illustrated in detail.

Fig. 2:

Sections of (Pn)₆ ring strands in PbP₇ (a) and Pb₂As_3.7(1)P_10.3(1). (b) The (Pn)₆ rings are emphasized in orange to distinguish them from the bridging atoms Pb in red, cis-P in blue and trans-Pn in black.

In 1-position either two trans-Pn or one trans-standing Pb1 is realized while in 4-position the opposite is true (Fig 2). Two cis-standing Pn and one cis-Pb can be found. Exactly this stereochemistry of the bridging atoms is responsible for the two-dimensional arrangement of the polypnictide substructure. Two of such (Pn)₆ ring strand units are attached to each other sharing the bridging atoms. In PbP₇, a three-dimensional polypnictide substructure is realized with a different stereochemistry for the bridging sites. Herein, alternating Pb, cis-P and trans-P sites are present. Due to the opposite orientation of the bridging P sites in PbP₇, a three-dimensional connectivity for the anionic substructure results. At this point the stereochemistry of such (Pn)₆ strands in black phosphorus and grey arsenic should be recalled. In black phosphorus, such strands are only trans-connected while in grey arsenic cis-connection is realized. Therefore, PbP₇ and Pb₂As_3.7(1)P_10.3(1) are derivatives of both element structures but with different stereochemical substitution patterns.

P and Pb are connecting the neighboring Pn layers. In the case of Pb₂As_3.7(1)P_10.3(1) only Pb2 bridges the two-dimensional polypnictide substructures.

For better comparison, in Fig 3 we show structure sections of black phosphorus, PbP₇, and Pb₂As_xP_14–x. The close relationship becomes obvious if the polypnictide substructure is compared. Choosing the right projection for each structure, the resulting polypnictide substructure seems to be almost identical. To illustrate the differences we draw three layers of black P and the respective polypnictide substructures of PbP₇ and Pb₂As_xP_14–x in different colors (turquoise, blue and green). By cutting 3-bonded P atoms out of the black phosphorus layers, 2-bonded ones with a formal charge of –1 result. These 2-bonded P atoms and some lone pairs of neighboring P atoms are coordinated by Pb²⁺.

Fig. 3:

Structure sections of (a) orthorhombic black phosphorus (view along the a axis, structure data taken from refs. [28, 29]), (b) PbP₇ (view along the a axis, data from ref. [3]) and the title compound Pb₂As_3.7(1)P_10.3(1) (right, view along the a axis). Rows of condensed (Pn)₆ rings are emphasized with an orange ellipse. All pnictide sites are drawn in turquoise, blue and green to illustrate the relationship between the different polypnictide substructures and to emphasize their differences; lead is drawn in red. Black spheres represent phosphorus positions connecting the black phosphorus-related polyphosphide layers in PbP₇.

Within a Zintl-precise formulation four of fourteen Pn atoms have two (oxidation number –1) and ten have three (oxidation number 0) Pn neighbors. Exactly two Pb²⁺ cations are needed for charge balance.

If we formulate a stacking sequence for the polypnictide substructure in PbP₇ a sequence A A A is realized while in Pb₂As_3.7(1)P_10.3(1) the sequence is A B A. The B layer turned by 180° relative to A and a slight translation are present. In Fig 3, a vertical line illustrates the translation. This small translation in combination with A B stacking renders an inversion center impossible substantiating the non-centrosymmetric space group.