La₂FSeTaO₄: lanthanum fluoride selenide oxotantalate or lanthanum fluoride oxoselenotantalate?

1 Introduction

For the ternary system LaF₃/La₂Se₃ two different compounds are known: LaFSe and La₂F₄Se [1], [2], [3], [4], [5]. The first one is dimorphic with tetragonal LaFSe (a ≈ 413.8 pm, c ≈ 715.2 pm) [4] as the room-temperature polymorph adopting the PbFCl-type structure, and hexagonal LaFSe (a ≈ 421.7 pm, c ≈ 818.8 pm) [2, 4] as a quenchable high-temperature form with a prototypic crystal structure. With V _m = 36.88 cm³ mol⁻¹, tetragonal LaFSe, called A-type LaFSe in the following, is the slightly more dense modification as compared to hexagonal (or A′-type) LaFSe with V _m = 36.97 cm³ mol⁻¹, although La³⁺ shows the same coordination number (C.N. = 9) in both polymorphs. The mixed-anionic distribution with four F⁻ and five Se²⁻ anions in A-LaFSe versus three F⁻ and six Se²⁻ anions in A′-LaFSe makes the difference, however. In trigonal La₂F₄Se [4] (a ≈ 417.9 c ≈ 2326.9 pm) with Ce₂F₄Se-type structure [5] there are ten anionic ligands (seven F⁻ and three Se²⁻ anions) coordinating the unique La³⁺ cations, so this arrangement serves as a missing link between tysonite-type LaF₃ (trigonal: a ≈ 719.0, c ≈ 736.7 pm) [6, 7] with eleven F⁻ anions for each La³⁺ cation and cation-deficient Th₃P₄-type La₂Se₃ (cubic: a ≈ 905.5 pm) [8] exhibiting eight Se²⁻ anions in the La³⁺ coordination sphere.

By adding oxide-anion contributions to the LaF₃/La₂Se₃ system, mixed-anionic quaternary compounds with three different anions (F⁻, O²⁻ and Se²⁻) become possible, which are interesting candidates for host materials of lanthanoid(III)-luminescent phosphors. Examples for these systems are dimorphic La₂OF₂Se [9], [10], [11] (trigonal: a ≈ 418.1, c ≈ 4478.2 pm; hexagonal: a ≈ 1396.8, c ≈ 401.1 pm) and La₆O₂F₈Se₃ [10] (hexagonal: a ≈ 1394.3, c ≈ 403.0 pm). Whenever oxygen is involved in the starting materials of a reaction mixture, tantalum containers as reaction vessels become vulnerable, manifested in the formation of lanthanum-oxotantalate derivatives, e. g. La₂Ta₃Se₂O₈ [12], La₂FSeTaO₄ or La₃F₂Se₂TaO₄ [13] as by-products.

The present contribution is well placed in a special issue dedicated to the 60th birthday of Martin Lerch, who studied the crystal structures and the properties of several new mixed-anionic tantalum compounds. Especially his article about the synthesis and crystal structure of metastable γ-TaON in Angewandte Chemie has so far been cited more than 55 times [14]. By adding europium as a rare-earth element to the tantalum oxide-nitride system he found pyrochlore-type Eu₂Ta₂(O,N)_7+δ and measured its magnetic properties [15]. Furthermore, he described rutile-type ScTa₂O₅N [16]. By replacing O²⁻ with N³⁻ and Ba²⁺ with La³⁺ in Ba₃Ta₅O₁₄N [17], he managed to synthesize LaBa₂Ta₅O₁₃N₂, a quinary compound containing lanthanum, tantalum and two different anions, so no better follow-up can be imagined as a contribution in honor of Martin Lerch.

2 Results and discussion

The new compound La₂FSeTaO₄ was found as a by-product from the synthesis of lanthanum oxide fluoride selenides (La₂OF₂Se [9], [10], [11] or La₆O₂F₈Se₃ [10]) in sealed tantalum capsules. It is the first representative with this composition and crystal structure, but already the second known compound, which contains the components lanthanum, fluorine, selenium, tantalum and oxygen. As the first quinary representative of these five elements, La₃F₂Se₂TaO₄ [13] is known to crystallize with the La₃F₂Se₂NbO₄-type structure [18]. La₂FSeTaO₄ shows a strong structural and metrical relationship to the representatives of the formula Ln ₃F₂SeTaO₄ (Ln = La–Nd) [13]. With a = 1146.51(6), b = 395.16(2) and c = 1281.32(7) pm for La₂FSeTaO₄, it has nearly the same a- and b-axes as La₃F₂Se₂TaO₄ [13] with a = 1132.59(6), b = 399.84(2) and c = 1811.72(9) pm. In fact the a- and b-axes of both compounds differ only by 1.2%, while the c-axis of the title compound is only about ²/₃ of the previously reported one. Moreover, both compounds crystallize in the orthorhombic space group type Pnma with Z = 4. The crystallographic data of La₂FSeTaO₄ is summarized in Table 1, while the fractional atomic coordinates and the equivalent isotropic displacement parameters (U _eq) can be taken from Table 2. Figure 1 shows six unit cells of La₂FSeTaO₄ viewed along [010] alongside with four unit cells of La₃F₂Se₂TaO₄ [13]. La₂FSeTaO₄ and La₃F₂Se₂TaO₄ [13] exhibit some striking structural similarities. Thus, La₃F₂Se₂TaO₄ can be written formally as La₂FSeTaO₄ ⋅ LaFSe. While La₂FSeTaO₄ contains two crystallographically different La³⁺ cations, but only one F⁻ and one Se²⁻ anion each, the LaFSe-richer compound La₃F₂Se₂TaO₄ [13] has one more of all of them. The coordination sphere of (La1)³⁺ is the same in both compounds, and the second one, (La2)³⁺, differs only by an oxide anion in La₂FSeTaO₄, which is replaced by a fluoride anion in La₃F₂Se₂TaO₄. The (La3)³⁺ cation, which is missing in La₂FSeTaO₄, is present in La₃F₂Se₂TaO₄ and shows the same coordination sphere as the formally missing LaFSe [4] in its tetragonal A-type polymorph. All La³⁺ coordination spheres of nine anions can be described as tricapped trigonal prisms [(La1)O₅FSe₃]¹⁴⁻ and [(La2)O₅F₂Se₂]¹³⁻ (Figure 2, top), which are connected by a triangular face of two Se²⁻ anions and the (O4)^2– anion. From a different point of view, the polyhedra can be described as capped square antiprisms, while in La₂FSeTaO₄ the capped square antiprisms of anions around the lanthanum cations create just dinuclear units (Figure 2, bottom), they form trinuclear units in La₃F₂Se₂TaO₄ [13] or even strands in the fluoride-free La₂Se₂Ta₃O₈ [12]. All La³⁺–Se^2– distances in La₂FSeTaO₄ are with 315 pm in average a little longer than in La₃F₂Se₂TaO₄ [13] with 312 pm. The mean La³⁺–O^2– and La³⁺–F^– distances in La₂FSeTaO₄ amount to 233 and 242 pm, respectively. The tantalum(V) cations in both La₂FSeTaO₄ and La₃F₂Se₂TaO₄ [13] are octahedrally surrounded by five O²⁻ anions and one Se²⁻ anion each. The [TaO₅Se]^7– octahedra build up chains along [010] by trans-oxygen corner-connection (Figure 3) in both cases with O4 as the linking oxygen atom. In the fluoride-free compound La₂Se₂Ta₃O₈ [12] the situation is different, because it features two crystallographically independent tantalum atoms, which are octahedrally surrounded. The first octahedron consists of six oxygen atoms, which form a double chain by trans-corner connectivity within the chains and edge-connectivity between the chains, but the second one exhibits two trans-oriented oxygen atoms and four selenium atoms building up a chain by selenium trans-edge connectivity. The major difference, however, results from the tantalum charge, as Ta⁵⁺ cations center the oxygen polyhedra [TaO₆]^7–, whereas Ta⁴⁺ cations are featured by the [TaO₂Se₄]^8– ions in La₂Se₂Ta₃O₈ [12]. The distances between tantalum and oxygen in both fluoride-containing compounds are nearly the same (195 pm, see Table 3 for details), but d(Ta⁵⁺–Se^2–) is only 292 pm long in La₃F₂Se₂TaO₄ [13] as compared to 317 pm in La₂FSeTaO₄. So the tantalum coordination sphere in La₃F₂Se₂TaO₄ [13] can be assigned C.N. = 6, but in La₂FSeTaO₄ only C.N. = 5 + 1. The Effective Coordination Number (ECoN after Hoppe [19, 20]) of Ta⁵⁺ with respect to Se²⁻ in La₂FSeTaO₄ is only 0.04, while it amounts to 0.21 in La₃F₂Se₂TaO₄ [13]. The ECoNs of tantalum with respect to oxygen range in both compounds between 0.7 and 1.3. Hence they are very similar to those in monoclinic LaTaO₄ [21, 22], again with values from 0.7 to 1.3 (Table 4). In La₂Se₂Ta₃O₈ [12] the first tantalum atom (Ta⁵⁺ only coordinated by oxide anions) shows ECoN values between 0.5 and 1.3 plus one deviant O²⁻ with 0.1, and the second one (Ta⁴⁺ surrounded by two oxygen and four selenium atoms) has ECoN values between 0.7 and 1.1 for Ta⁴⁺ against Se²⁻ and 1.1 with respect to O²⁻. While in La₂Se₂Ta₃O₈ [12] there is a strong attraction between Ta⁴⁺ and Se²⁻ in accord with Pearson’s HSAB concept [23], there are only weak Ta⁵⁺–Se^2– interactions in both fluoride-containing compounds. The ECoN values within the tantalum coordination polyhedra in selected compounds are compared in Table 4. While the ECoNs of Ta⁵⁺ in La₂FSeTaO₄ and La₃F₂Se₂TaO₄ [13] are very similar and described with values of about 5, geometric considerations reveal that tantalum is only surrounded square-pyramidally in La₂FSeTaO₄, but octahedrally in La₃F₂Se₂TaO₄ [13].

Figure 1:

Six unit cells of La₂FSeTaO₄ (left) and four unit cells of La₃F₂Se₂TaO₄ [13] (right) as viewed along [010].

Figure 2:

Coordination spheres of (La1)³⁺ and (La2)³⁺ in the crystal structure of La₂FSeTaO₄ as tricapped trigonal prisms (top) or as face-connected capped square antiprisms (bottom).

Figure 3:

Square-pyramidal surrounding of Ta⁵⁺ by oxygen atoms and the trans-vertex connection of the [TaO₅]^5– units along [010]. The resulting chains in La₂FSeTaO₄ show bridging O4 atoms and an isotactic orientation of the O1 pyramid tops. An extra Se²⁻ anion at a distance of 317 pm to Ta⁵⁺ in trans-orientation to O1 completes each coordination sphere pseudo-octahedrally.

The selenide anions are surrounded in the form of capped square pyramids by five La³⁺ cations with one additional tantalum cap. These [SeLa₅Ta]¹⁸⁺ polyhedra build a double chain along the [010] direction by edge-connections (Figure 4). Oxygen and fluorine atoms show the same tetrahedral and trigonal-planar surroundings as in La₃F₂Se₂TaO₄ [13]. The assignment of O²⁻ and F⁻ to the anions with the coordinates (0.438, ¹/₄, 0.718) and (0.426, ¹/₄, 0.153) was enabled by bond-valence calculations (Table 5). Both sites are coordinated by a triangle of La³⁺ cations (and an extra Ta⁵⁺ cation in the case of the (0.438, ¹/₄, 0.718) position), but one of them should be oxide and the other one fluoride, for charge-balance reasons. The bond-valence sum for fluoride at (0.426, ¹/₄, 0.153) is close to 1 and at (0.438, ¹/₄, 0.718) higher than 1.5, so fluoride should be at (0.426, ¹/₄, 0.153) and oxide at the other Wyckoff site. The cationic coordination polyhedra of the light anions F⁻ and O²⁻ can be seen in Figure 5. Selected interatomic distances of the title compound La₂FSeTaO₄ compared to the LaFSe-richer one La₃F₂Se₂TaO₄ [13] are summarized in Table 3. The motifs of mutual adjunction for La₂FSeTaO₄ as compared to those in La₃F₂Se₂TaO₄ [13] are shown in Table 6.

Figure 4:

Ta⁵⁺-capped [SeLa₅]¹³⁺ polyhedra form a double chain along [010] in La₂FSeTaO₄ by edge condensation.

Figure 5:

Cationic triangular and tetrahedral surroundings of the fluoride and oxide anions in La₂FSeTaO₄.

The quinary compound La₃F₂Se₂TaO₄ [13] is formally consisting of La₂FSeTaO₄ and LaFSe, so the Madelung Part of Lattice Energy (MAPLE according to Hoppe [19, 20, 24]) of La₂FSeTaO₄ should be the one of La₃F₂Se₂TaO₄ [13] minus the one of LaFSe [4]. For LaFSe [4], there is a hexagonal (A′-type) and a tetragonal (A-type) polymorph, but the agreement of MAPLE for both with a difference of less than 0.6% is just striking. The detailed MAPLE values are given in Table 7 for comparison also with the summed up data of La₂Se₃ [25], LaF₃ [7] and LaTaO₄ [22], which are the compounds that can be found in the powder pattern of the product from attempts to synthesize La₂FSeTaO₄ as the target compound. With a 4% difference of these values it is not surprising, that the title compound La₂FSeTaO₄ or its LaFSe-richer homologue La₃F₂Se₂TaO₄ [13] can only be found as minor by-products through serendipitous attack of the tantalum capsules. A targeted synthesis had not yet been successful.

The answer to the question in the title of this article is that La₂FSeTaO₄ would be better described as lanthanum fluoride selenide oxotantalate than as lanthanum fluoride oxoselenotantalate. La₂FSeTaO₄ does not represent a new lanthanum fluoride selenide oxoselenotantalate as La₃F₂Se₂TaO₄ [13] before, since the latter contains two different selenide anions, namely one as a selenide anion to please La³⁺ and the other one as a ligand for Ta⁵⁺. The single selenide anion in La₂FSeTaO₄ tries to serve for both functions and becomes overtasked, as its distance to Ta⁵⁺ expands to 317 pm (ECoN = 0.05) compared to the dedicated one in La₃F₂Se₂TaO₄ [13] (294 pm, ECoN = 0.21). On the other hand, its role as a Se²⁻ ligand in the coordination sphere of the La³⁺ cations is indeed also strongly reduced (d(La³⁺–Se^2–) = 312–318 pm, d ‾  = 315 pm in La₂FSeTaO₄ versus d(La³⁺–Se^2–) = 307–315 pm, d ‾  = 312 pm in La₃F₂Se₂TaO₄ [13]). So La₂FSeTaO₄ should be rather addressed as a lanthanum fluoride selenide oxotantalate without selenium in the first tantalum coordination sphere, even if the trans-influence in the [TaO₅Se]^7– quasi-octahedra with 183 vs. 317 pm for the O1–Ta⋅⋅⋅Se line in La₂FSeTaO₄ appears not any stronger than the analogous one with 182 vs. 292 pm in La₃F₂Se₂TaO₄ [13].

3 Experimental

Single crystals of La₂FSeTaO₄ emerged as a by-product from solid-state reactions between La, LaF₃, La₂O₃ and Se designed to produce La₂OF₂Se [9], [10], [11] or La₆O₂F₈Se₃ [10]. The targeted synthesis of La₂FSeTaO₄ according to the equation

was not successful, but some colorless, lath-shaped single crystals of this compound were obtained as a by-product from reactions of lanthanum metal, lanthanum trifluoride, lanthanum sesquioxide (all ChemPur, Karlsruhe, 99.9%) and elemental selenium (Alfa Aesar, Karlsruhe, 99.999%) with a mass equivalent of sodium chloride (ChemPur, Karlsruhe, 99.96%) as fluxing agent in tantalum tubes (Sigma-Aldrich, Steinheim, without any special pretreatment). The tantalum tubes were filled in an argon glove box with the reactants and welded in an electric arc. The welded containers were transferred into tubes (Figure 6) of fused silica to protect them from air and heated for 7 days at 850 °C. Needle-shaped single crystals of the composition La₃F₂Se₂TaO₄ [13] were found in similar experiments, but never as a pure target compound along the following equation

The crystal structure was solved after an X-ray diffraction experiment of a selected single crystal with Mo-K _α radiation (λ = 71.07 pm) at ambient temperature with an IPDS-I diffractometer by STOE & Cie (Darmstadt). The structure determination succeeded with the program package Shelx-97 [26], [27], [28] and absorption correction with X-Shape [29]. The bond-valence calculations were made with Brese and O’Keeffe’s values [30] and for MAPLE calculations [19, 20] the software of Hübenthal and Hoppe [24] was used. The same software found also application for calculation of the Effective Coordination Numbers (ECoN).

Figure 6:

Example of two filled and arc-welded tantalum capsules inside a fused silica tube.

4 Conclusions

From reactions of mixtures of lanthanoid metals, lanthanoid trifluorides, their sesqiuoxides, and elemental selenium in a sodium chloride flux at 850 °C in sealed tantalum capsules products like Ln ₂OF₂Se [9], [10], [11], Ln ₃OFSe₃ [31], Ln ₃OF₃Se₂ [31], Ln ₅OF₅Se₄ [32], [33], [34], Ln ₆O₂F₈Se₃ [10] and Ln ₆O₄F₄Se₃ [35] (Ln = La–Ho) can be obtained as lanthanoid oxide fluoride selenides. In the case of lanthanum and with the use of tantalum tubes not subjected to any pretreatment, colorless needle-shaped single crystals of orthorhombic La₂FSeTaO₄ and La₃F₂Se₂TaO₄ [13] occurred as by-products. La₃F₂Se₂TaO₄ [13] as the first representative of this kind has a structure which is not only an arithmetic intergrowth of La₂FSeTaO₄ with LaFSe. In the new compound, the two crystallographic different La³⁺ cations are coordinated by nine anionic ligands in the shape of capped square antiprisms or tricapped trigonal prisms. The tantalum atom is surrounded by five oxygen atoms as a square pyramid and an additional selenium atom at a longer distance of 317 pm in La₂FSeTaO₄ and at 292 pm in La₃F₂Se₂TaO₄ [13] as a sixth neighbor. The [TaO₅]^5– units are trans-corner connected and build a chain along [010]. While Se²⁻ is coordinated by five La³⁺ and one Ta⁵⁺ as an extra cap of the square pyramid, the four different oxygen and the fluorine atoms in La₂FSeTaO₄ are coordinated in a triangular or tetrahedral fashion. The oxide and fluoride sites were individually determined by bond-valence calculations. The MAPLE value of La₂FSeTaO₄ added to that of LaFSe [4] shows a good agreement with the one of La₃F₂Se₂TaO₄ [13] with a difference of only 0.2% for hexagonal A′-LaFSe [4] and 0.3% for the tetragonal A-LaFSe [4].