RbFe(HAsO4)2 and TlFe(HAsO4)2, two new hydrogenarsenates adopting two closely related structure types

Rubidium iron bis[hydrogen arsenate(V)] and thallium iron bis[hydrogen arsenate(V)] were grown under mild hydrothermal conditions (T = 493 K, 7 d). RbFe(HAsO4)2 adopts the RbFe(HPO4)2 structure type (space group R c), while TlFe(HAsO4)2 crystallizes in the (NH4)Fe(HPO4)2 structure type (space group P ).


Chemical context
Compounds with mixed tetrahedral-octahedral (T-O) framework structures exhibit a broad range of different topologies, resulting in structures with various interesting properties. Arsenates, similar to phosphates or silicates, tend to form T-O framework structures, with properties such as ion conductivity (Chouchene et al., 2017;d'Yvoire et al., 1983d'Yvoire et al., , 1986d'Yvoire et al., , 1988Masquelier et al., 1990Masquelier et al., , 1994Masquelier et al., , 1995Masquelier et al., , 1996Masquelier et al., ,1998Ouerfelli et al., 2007aOuerfelli et al., , 2008Pintard-Scré pel et al., 1983) and ion exchange (Masquelier et al., 1996), as well as unusual piezoelectric (Cambon et al., 2003(Cambon et al., , 2005Krempl, 2005;Ren et al., 2015), magnetic (Ouerfelli et al., 2007b) or non-linear optical features (frequency doubling) (Carvajal et al., 2005;Kato, 1975;Sun et al., 2017). To further increase the knowledge about the possible compounds and structure types of arsenates, a comprehensive study of the system M + -M 3+ -O-(H)-As 5+ (M + = Li, Na, K, Rb, Cs, Ag, Tl, NH 4 ; M 3+ = Al, Ga, In, Sc, Fe, Cr, Tl) was undertaken, which led to a large number of new compounds, most of which have been published (Schwendtner & Kolitsch, 2004. Among the many different structure types found during our study, one atomic arrangement, the RbFe(HPO 4 ) 2 type (Lii & Wu, 1994;rhombohedral, R3c), was found to be extremely versatile, allowing the incorporation of a wide variety of cations. Representatives of this structure type are presently known among arsenates and phosphates containing Rb or Cs as the M + cation and Al, Ga, Fe, In as M 3+ ; see Table 1 for a complete compilation of these compounds. RbFe(HAsO 4 ) 2 (Fig. 1a) is the fifth arsenate adopting this structure type. There is only one other Rb-Fe-arsenate known to date, Table 1 Compilation of all published compounds adopting the (NH 4 )Fe(HPO 4 ) 2 structure type (Yakubovich, 1993) and the RbFe(HPO 4 ) 2 structure type (Lii & Wu, 1994). (NH 4  Tl2-O bond length in TlFe(HAsO 4 ) 2 (3.312 Å ) is the longest average bond length found so far for TlO 12 polyhedra (max. Tl-O = 3.304 Å ; Gagné & Hawthorne, 2018) and the corresponding average Rb2-O bond length in RbFe(HAsO 4 ) 2 is also close to the longest observed such bond lengths in RbO 12 polyhedra of 3.410 Å (Gagné & Hawthorne, 2016). These loose bonds reflect the observation that the alkali cations 'rattle' somewhat in their hosting voids, with considerable positional disorder of the Tl atoms in these voids (Fig. 4b). The Tl atoms were therefore modelled with two Tl1 positions (Tl1A, Tl1B) and three Tl2 positions (Tl2A, Tl2B, Tl2C), between 0.28 (2) and 0.48 (2) Å apart. The refined occupancies of the dominant positions (Tl1A and Tl2A) are 63 and 45%, respectively. The influence of a stereochemically active lone pair of electrons on the Tl + cations may also play a role in the positional disorder. The average Fe-O bond lengths, which show a fairly narrow range between 1.998 and 2.006 Å for the four FeO 6 octahedra in the two title compounds, are slightly lower than the corresponding grand mean average of 2.011 Å reported by Baur (1981), thus leading to slightly higher BVSs of between 3.11 and 3.15 v.u. (Gagné & Hawthorne, 2015).
The AsO 4 tetrahedra are distorted with three short bond lengths of those bonds connecting to neighbouring FeO 6 octahedra and one considerably elongated bond length to the protonated corner.   to the calculated average of 1.686 (10) Å (calculated on 704 AsO 4 polyhedra; Schwendtner, 2008), and the two As-OH bond lengths (Tables 3 and 4) are also close to the average of such lengths in HAsO 4 polyhedra of 1.72 (3) Å (Schwendtner, 2008), but the two bond lengths to O atoms with rather strong hydrogen bonds [DÁ Á ÁA = 2.569 (3) and 2.615 (3) Å ] are considerably elongated to 1.738 (2) and 1.742 (2) Å , respectively (Tables 2 and 3).
were washed thoroughly with distilled water, filtered and dried at room temperature. They are stable in air.
The chemical compositions of the title compounds were checked by standard SEM-EDS analysis of several carboncoated crystals of each compound; no impurities could be detected.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 6.
For the final refinement the atomic positions of RbFe(HPO 4 ) 2 (Lii & Wu, 1994) and CsSc(HAsO 4 ) 2 (Schwendtner & Kolitsch, 2004) were used for RbFe(HAsO 4 ) 2 and TlFe(HAsO 4 ) 2 , respectively. The H atoms were then located from the difference-Fourier map and O-H distances were restrained to 0.90 (4) Å . The position of H8 was fixed to the coordinates where it was located in the difference-Fourier map, since a refinement of the position led to an unreasonably close distance to the neighbouring As atom. At this point, electron densities of up to 2.79 and 4.71 e Å À3 , respectively, were found close to the Tl1 and Tl2 atoms, along with anomalous displacement ellipsoids of these atoms. This suggested the presence of positional disorder (and, possibly, some mobility) of the Tl atoms in the cavities. The disorder was then modeled by additional, partially occupied Tl positions. The bulk occupancy for each of the two disordered Tl positions (Tl1A and Tl1B for Tl1 and Tl2A, Tl2B and Tl2C for Tl2) was constrained to 1.00. As a result, the R value dropped from 0.0335 to 0.0224, and the weight parameters also improved. Final equivalent isotropic displacement parameters of all the partially occupied Tl sites are reasonable, with values between ca 0.03 and 0.04 Å 2 , very similar to those in the Rb compound. The final residual electron densities are < 1 e Å À3 for both compounds.  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) for RbFeHAsO42; WinGX (Farrugia, 2012) for TlFeHAsO42.

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.