CsGa(HAsO4)2, the first Ga representative of the RbAl(HAsO4)2 structure type

The tetrahedral-octahedral framework crystal structure of hydrothermally synthesized CsGa(HAsO4)2 was solved by single-crystal X-ray diffraction. CsGa(HAsO4)2 crystallizes in the polar RbAl(HAsO4)2 structure type (R32).


Chemical context
Compounds with mixed tetrahedral-octahedral (T-O) framework structures are characterized by a broad range of different atomic arrangements. These topologies result in several interesting properties such as ion exchange (Masquelier et al., 1996) and ion conductivity (Chouchene et al., 2017), as well as unusual piezoelectric (Ren et al., 2015), magnetic (Ouerfelli et al., 2007) or non-linear optical features (frequency doubling; Sun et al., 2017).

Figure 1
General outline of the crystal structure of CsGa(HAsO 4 ) 2 viewed along a.
Only the main AsO 4 tetrahedra are shown (the AsB-centred tetrahedra are omitted for clarity). Hydrogen bonds are shown as blue dotted lines.  layers are held together by medium-strong hydrogen bonds (Table 2). Nearly all of the representatives of these closely related structure types show pseudo-hexagonal to pseudooctahedral crystal habits. In line with this observation, CsGa(HAsO 4 ) 2 forms tiny pseudo-hexagonal platelets. The two Cs atoms in the framework voids are 12-coordinated. While the average Cs1-O bond length, 3.395 Å , is slightly longer than the grand mean average of 3.377 Å (Gagné & Hawthorne, 2016), it fits the low bond-valence sum (BVS) of 0.84 valence units (v.u.) which was calculated with the bond-valence parameters of Gagné & Hawthorne (2015). In contrast, the average Cs2-O bond length is slightly shorter (3.359 Å ) and the individual Cs2-O bond lengths (Table 2) show a much wider bond-length range, resulting in a much too high bond-valence sum of 1.38 v.u. This is mainly caused by four very short Cs2-O bond lengths of only 3.014 Å , although even shorter Cs-O bond lengths, as low as 2.910 Å , have been reported for 12-coordinated Cs + cations (Gagné & Hawthorne, 2016).
The Ga atoms at the centre of the two GaO 6 octahedra are also slightly overbonded with BVSs of 3.05 and 3.07 v.u., and average Ga-O bond lengths of 1.970 and 1.967 Å for Ga1 and Ga2, respectively. These values are somewhat shorter than the grand mean average for six-coordinated Ga of 1.978 Å (Gagné & Hawthorne, 2018). The AsO 4 tetrahedra show the typical bond-length geometry of HAsO 4 groups with three short and one long As-O bond. The average As-O bond length (1.689 Å ) is very close to the observed average of HAsO 4 groups (1.687 Å ; Schwendtner & Kolitsch, 2019), but the As-O bond length to the protonated O4 atom (1.740 Å , Table 2) is notably longer than the average of 1.728 Å for As-OH bonds in singly protonated AsO 4 groups (Schwendtner & Kolitsch, 2019). The BVS for the As atom is close to ideal with 4.98 v.u. All its O ligands are underbonded to a varying degree, with BVSs ranging from 1.39 v.u. for O4 to 1.92 v.u. for O1.
The As atom is characterized by a split position. The AsB site, 1.27 Å away from the main As position, has a refined occupancy of about 5%. The AsB site shares one apical ligand (O1) with the main AsO 4 tetrahedron and has three additional low-occupancy O atoms (O2B, O3B and O4B) as remaining ligands. The split position can roughly be explained by a mirror plane in (110). The average AsB-O bond length of 1.684 Å is slightly shorter than the corresponding value of the main AsO 4 tetrahedron (1.689 Å ), and the AsB-O bonds also show a wider bond-length range ( Table 2). The calculated BVS for the AsB site (5.09 v.u.) is reasonable considering the high estimated uncertainty of this value in view of the relatively large positional and bond-length errors for the AsB site (Table 2).

Synthesis and crystallization
Small pseudo-hexagonal colorless platelets of CsGa(HAsO 4 ) 2 were prepared hydrothermally (T = 493 K, 7 d) in a Teflonlined stainless steel autoclave from a mixture of Cs 2 CO 3 , Ga 2 O 3 (approximate molar ratio Cs:Ga of 1:1), arsenic acid and distilled water. Enough arsenic acid was added to keep the pH between about 1.5 and 0.5. The Teflon cylinders were filled with distilled water up to approximately 80% of their inner volume. Initial and final pH values were about 1.5 and 1, respectively. The platelets were accompanied by large colourless glassy prisms of CsGa(H 2 AsO 4 )(H 1.5 AsO 4 ) 2 (Schwendtner & Kolitsch, 2005), which made up about 80% of the reaction products.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3.
The refinement of CsGa(HAsO 4 ) 2 revealed a considerable residual electron-density peak of 5.1 e Å À3 1.27 Å away from As and 1.62 Å away from the O1 site. The corresponding position can be generated by a mirror plane in (110) and therefore was assumed to be an alternative flipped As position (sharing the same O1 atom), similar to what was encountered in related TlAl(HAsO 4 ) and CsIn(HAsO 4 ) 2 (R3c type; Schwendtner & Kolitsch, 2017b). An inclusion of the alternative position led to a considerable drop in the conventional R factor and weight parameters and the highest residual electron densities also decreased considerably. Three electron-density peaks between 1.15 and 1.19 e Å À3 close to this AsB position could be attributed to the O ligands of this flipped AsO 4 tetrahedra and, after including them into the  structure model, the conventional R factor dropped from 3.5 to 1.99%. The remaining highest residual electron densities of 0.72 and À0.74 e Å À3 are located close to the Cs positions. The occupancy of the alternative As position (Fig. 2) refined to about 5%, while the independently refined occupancy of the main As position was about 95%. For the final refinement, the displacement parameters of the AsB, O2B, O3B and O4B sites were restrained to be the same as that of the main AsO 4 tetrahedron position, and the occupancy sums of both tetrahedra were restrained to give a total occupancy of 1.00. The structure was refined as inversion twin with a Flack parameter of 0.46 (2). Data collection: COLLECT (Nonius, 2003); cell refinement: HKL SCALEPACK (Otwinowski et al., 2003); data reduction: HKL DENZO and SCALEPACK (Otwinowski et al., 2003); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2016 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 2005); software used to prepare material for publication: publCIF (Westrip, 2010).