(NH4)Ga(HAsO4)2 and TlAl(HAsO4)2 - two new RbFe(HPO4)2-type M + M 3+ arsenates

The crystal structures of hydrothermally synthesized (NH4)Ga(HAsO4)2 and TlAl(HAsO4)2 were solved by single-crystal X-ray diffraction. They both crystallize in the common RbFe(HPO4)2 structure type (R c).


Chemical context
Compounds with mixed tetrahedral-octahedral (T-O) framework structures feature a broad range of different atomic arrangements. These result in topologies with 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).

Structural commentary
The two compounds are representatives of the RbFe(HPO 4 ) 2 structure type (R3c; Lii & Wu, 1994) and show a basic tetrahedral-octahedral framework structure featuring inter- Structure drawings of the framework structures of (a) (NH 4 )Ga(HAsO 4 ) 2 and (b) TlAl(HAsO 4 ) 2 viewed along a. The unit cell is outlined and the alternative position AsB in (b) is shown in light yellow (the main As position is orange). The Tl1 atom shows a slight positional disorder and is slightly offset from the ideal position.

Figure 2
Structure drawings of the framework structures of (a) (NH 4 )Ga(HAsO 4 ) 2 and (b) TlAl(HAsO 4 ) 2 viewed along c. The unit cells are outlined and the alternative position AsB in (b), which can be generated by a mirror plane in (110), is shown in light yellow (the main As position is orange). The Tl1 atom shows a slight positional disorder.

Synthesis and crystallization
The compounds were grown by hydrothermal synthesis at 493 K (autogeneous pressure, slow furnace cooling) using Teflon-lined stainless steel autoclaves with an approximate filling volume of 2 cm 3 . Reagent-grade NH 4 OH, Tl 2 CO 3 , Ga 2 O 3 , Al 2 O 3 and H 3 AsO 4 Á0.5H 2 O were used as starting reagents in approximate volume ratios of M + :M 3+ :As of 1:1:3 of the respective M + M 3+ compound for both synthesis batches. For TlAl(HAsO 4 ) 2 , the vessels were filled with distilled water to about 70% of their inner volumes, which led to initial and final pH values of 1 and 0.5, respectively, and the synthesis was allowed to proceed at 493 K for 9 d. (NH 4 )Ga(HAsO 4 ) 2 was grown over a period of 7 d and the initial and final pH values were 3 and 1, respectively. The reaction products were washed thoroughly with distilled water, filtered, and dried at room temperature. (NH 4 )Ga(HAsO 4 ) 2 formed large colourless pseudo-octahedral crystals (Fig. 3), while TlAl(HAsO 4 ) 2 formed small pseudo-hexagonal platelets. Both compounds are stable in air.
Semiquantitative SEM-EDX analysis (15 kV) of carboncoated, horizontally oriented crystals of (NH 4 )Ga(HAsO 4 ) 2 were undertaken to discriminate between H 3 O + and NH 4 + . They confirmed the suspected formula and revealed no impurities.

Refinement
Crystal data, data collection, and structure refinement details are summarized in Table 5.
For the refinement of both compounds, the coordinates of RbFe(HPO 4 ) 2 (Lii & Wu, 1994) were used for the initial refinement steps. The hydrogen atoms were then located in difference-Fourier maps and added to the models. In both compounds O-H bonds were restrained to 0.9 AE 0.04 Å . In (NH 4 )Ga(HAsO 4 ) 2 , several electron-density peaks between   0.4 and 0.75 e Å À3 were recognizable that could be attributed to the H atoms of the NH 4 + cation. These peaks are located at the following coordinates for the N1 atom: 0.0170, 0.1329, 0.7450; 0.0641, 0.0560, 0.7414 and À0.0910, 0.0000, 0.7500. For the N2 atom, the coordinates are: 0.0478, À0.0330, 0.6635; À0.0655, À0.1106, 0.6786; 0.1301, 0.0094, 0.6695 and À0.0521, À0.0657, 0.6513. However, despite the use of restraints, no sensible coordination geometry for the H atoms around the N atoms could be found. Therefore, they were omitted from the model. As a result of the fact that there are 12 possible N-HÁ Á ÁO bonds for each N atom, with only two symmetryequivalent positions for N1 and four for N2, it seems reasonable to assume that the H-atom positions around the N atoms are, in both cases, highly disordered. The final residual electron density in (NH 4 )Ga(HAsO 4 ) 2 is < 1e Å À3 .
The refinement of TlAl(HAsO 4 ) 2 revealed a considerable residual electron-density peak of 2.2 e Å À3 1.28 Å away from As and 1.61 Å away from the O1 site. The corresponding position can be generated by a mirror plane in (110) and therefore could be an alternative flipped As position (sharing the same O1 atom). Since the inclusion of the alternative position led to a considerable drop in R 1 and weighting parameters and the highest residual electron density dropped to < 1 e Å À3 , this position was kept in the model. The occupancy of the alternative position AsB (Fig. 1b, 2b) refined to only 2.1%, which makes it impossible to locate the alternative O ligand positions that should comprise the coordination sphere of the AsB position. For the final refinement, the displacement parameters of the AsB position were restrained to be the same as for the main As position and the sum of As was restrained to give a total occupancy of 1.00. We note that a similar alternative position was also found for isotypic CsIn(HAsO 4 ) 2 (Schwendtner & Kolitsch, 2017b).
There was also considerable residual electron density of AE2 e Å À3 close to the two Tl positions, similar to what was encountered in the structurally related TlGa 2 As(HAsO 4 ) 6 (Schwendtner & Kolitsch, 2018d). We tried a similar approach that had worked well for the aforementioned compound, viz. to remove the Tl atoms from their ideal, highly symmetrical positions in this structure type. We obtained a better refinement with a slightly off-centre position for Tl1, in line with a slight disorder (probably static), possibly in part or in whole due to the stereochemical activity of the lone electron pair on the Tl + cations. So, although the Tl1 site is slightly offset from its ideal position (0, 0, 3/4), we unfortunately did not manage to get rid of the negative residual electron density of about À2 e Å À3 next to Tl2. The most positive residual electron density peak, however, dropped to < 1 e Å À3 .

Funding information
Funding for this research was provided by: Doc fForte Fellowship of the Austrian Academy of Sciences to K. Schwendtner. The authors acknowledge the TU Wien University Library for financial support through its Open Access Funding Program.

Ammonium gallium bis[hydrogen arsenate(V)] (NH4GaHAsO42)
Crystal data (NH 4   Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.