Two new Rb–Ga arsenates: RbGa(HAsO4)2 and RbGa2As(HAsO4)6

The crystal structures of hydrothermally synthesized RbGa(HAsO4)2 and RbGa2As(HAsO4)6 were solved by single-crystal X-ray diffraction. They both crystallize in related R c structure types, one of which contains AsO6 octahedra assuming the topological role of M 3+O6 octahedra.


Chemical context
Compounds with mixed tetrahedral-octahedral (T-O) framework structures feature a broad range of different atomic arrangements, resulting in topologies with various 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 nonlinear optical features (frequency doubling) (Sun et al., 2017). In order to further increase the insufficient knowledge about the crystal chemistry 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, 2007, 2018a. Among the many different structure types found during our study, one atomic arrangement, i.e. the RbFe(HPO 4 ) 2 type (Lii & Wu, 1994;rhombohedral, R3c), was found to show a large crystal-chemical flexibility which allows the incorporation of a wide variety of cations. A total of nine representatives of this structure type are presently known among M + M 3+ (HTO 4 ) 2 (T = P, As) compounds containing Rb or Cs as the M + cation and Al, Ga, Fe or In as the M 3+ cation (Lesage et al., 2007;Lii & Wu, 1994;Schwendtner & Kolitsch, 2017, 2018a, including RbGa(HPO 4 ) 2 (Lesage et al., 2007). One of the title compounds, RbGa(HAsO 4 ) 2 , is another new representative of the RbFe(HPO 4 ) 2 structure type. The second title compound, RbGa 2 As(HAsO 4 ) 6 , is the third representative of a recently described variation of the RbFe(HPO 4 ) 2 type, the RbAl 2 -As(HAsO 4 ) 6 type. It also crystallizes in R3c and up to now members with RbAl and CsFe as M + M 3+ cation combinations ISSN 2056-9890 are known (Schwendtner & Kolitsch, 2018b). Interestingly, all presently known M + M 3+ combinations adopting this new structure type also have representatives adopting the RbFe(HPO 4 ) 2 type. It thus seems likely that more of the known RbFe(HPO 4 ) 2 -type arsenates would also adopt the new RbAl 2 As(HAsO 4 ) 6 -type atomic arrangement under formally 'dry' synthesis conditions (see x3). RbGa 2 As-(HAsO 4 ) 6 is a rare example of a compound containing AsO 6 octahedra. Out of all reported arsenates(V), only about 3% contain AsO 6 polyhedra, according to our earlier review paper (Schwendtner & Kolitsch, 2007), which provides an overview of all known compounds containing AsO 6 groups and their bond-length statistics. At present, 37 compounds containing As in an octahedral coordination are known (Schwendtner & Kolitsch, 2018b); RbGa 2 As(HAsO 4 ) 6 represents the 38 th member of this class of compounds. While 12 Rb-and Gacontaining phosphates are contained in the ICSD (FIZ, 2018), only one Rb-Ga arsenate, i.e. RbGaF 3 (H 2 AsO 4 ) (Marshall et al., 2015), is known so far. Since submitting this paper, another paper dealing with isotypic M + M 3+ 2 As(HAsO 4 ) 6 compounds (M + M 3+ = TlGa, CsGa, CsAl) has been published (Schwendtner & Kolitsch, 2018c).

Figure 1
Crystal structure drawings of (a) RbGa 2 As(HAsO 4 ) 6 and (b) RbGa-(HAsO 4 ) 2 in views along the b axis. A part of the GaO 6 octahedra is replaced by AsO 6 octahedra in RbGa 2 As(HAsO 4 ) 6 ; the corresponding layers (see Figs. 2 and 3) are compressed along c and the corresponding void remains vacant of Rb atoms.
Since the unit-cell dimensions in directions a and b are slightly longer in RbGa 2 As(HAsO 4 ) 6 and the AsO 6 octahedra are smaller than the corresponding GaO 6 octahedra, the (Ga/As)As 6 O 24 units within this layer move further apart -leading to longer D-HÁ Á ÁA distances and a compressed (along c) void that is too small for Rb atoms (compare Fig. 1). also related to the triclinic (NH 4 )Fe(HPO 4 ) 2 type (P1; Yakubovich, 1993) and the RbAl(HAsO 4 ) 2 type (R32; Schwendtner & Kolitsch, 2018b). The fundamental building unit in all these structure types contains M 3+ O 6 octahedra which are connected via their six corners to six protonated AsO 4 tetrahedra, thereby forming an M 3+ As 6 O 24 unit. These units are in turn connected via three corners to other M 3+ O 6 octahedra. The free protonated corner of each AsO 4 tetrahedron forms a hydrogen bond to the neighbouring M 3+ As 6 O 24 group (Fig. 2). The M 3+ As 6 O 24 units are arranged in layers perpendicular to the c hex axis (Fig. 1). The units within these layers are held together by medium-strong hydrogen bonds (Tables 1 and 2). Both title compounds invariably show a very similar crystal habit: strongly pseudohexagonal to pseudo-octahedral (cf. Fig. 3). The new compound RbGa 2 As(HAsO 4 ) 6 could only be grown by 'dry' hydrothermal techniques (without the addition of water). The extreme abundance of As during the synthesis and the formation of a melt of arsenic acid promotes the formation of this novel structure type and endorses the octahedral coordination of As. The substitution of one third of all Ga 3+ cations by As 5+ requires that two thirds of all Rb + cations are omitted to achieve charge balance (compare Figs. 1a,1b,2a and 2b). This substitution also has an effect on the unit-cell parameters (Table 3) and the pore diameter. Since GaO 6 is only replaced by AsO 6 in every second layer (perpendicular to the c axis), the a axis must remain long enough to still be able to house the GaO 6 in the layers between. The effect of the smaller AsO 6 octahedra is therefore mainly reflected by a strong compression of about 5% along the c axis, while the a axis becomes even slightly longer compared to RbGa-(HAsO 4 ) 2 . Due to the comparatively smaller AsO 6 octahedra, the (Ga/As)As 6 O 24 units are further apart in RbGa 2 As-(HAsO 4 ) 6 and the encased void is compressed along c, making it too small to house Rb + cations ( Figs. 1 and 2). This effect is also reflected by the considerably elongated hydrogen bond in RbGa 2 As(HAsO 4 ) 6 . While these bonds, which connect neighbouring (Ga/As)As 6 O 24 groups, are very strong in RbGa(HAsO 4 ) 2 [D-HÁ Á ÁA = 2.598 (2) Å ], they are much longer in RbGa 2 As(HAsO 4 ) 6 [2.7314 (17) Å ; compare Tables  1 and 2]. The second layer, in contrast, remains practically identical in both compounds and contains Rb atoms with a slight positional disorder (Fig. 4). In both compounds, the Rb atoms are 12-coordinated (Figs. 2 and 3), and the average Rb-O bond lengths in RbGa 2 As(HAsO 4 ) 6 (3.433 Å ) are longer than the longest average bond length in RbO 12 polyhedra of 3.410 Å reported so far (Gagné & Hawthorne, 2016), thus leading to rather low bond-valence sums (BVSs; Gagné & Hawthorne, 2015) of only 0.59 valence units (v.u.), whereas the corresponding BVSs are 0.82 and 0.84 v.u. for RbGa(HAsO 4 ) 2. These loose bondings lead to considerable positional disorder of the Rb + cations in these voids, which were modelled with two Rb positions, between 0.41 (2) and 0.42 (4) Å apart. While position Rb1A in the centre of the large framework void in RbGa 2 As(HAsO 4 ) 6 has only 77% occupancy compared to the off-centre position Rb1B (with occupancy 23%), in RbGa-(HAsO 4 ) 2 , the central position Rb1A has 91% occupancy. Similar behaviour was observed for the isotypic CsFe and RbAl compounds (Schwendtner & Kolitsch, 2018b), as well as isotypic phosphates (Lesage et al., 2007).

Synthesis and crystallization
The compounds were grown by hydrothermal synthesis at 493 K (7 d, autogeneous pressure, slow furnace cooling) using Teflon-lined stainless steel autoclaves with an approximate filling volume of 2 ml. Reagent-grade Rb 2 CO 3 , Ga 2 O 3 and H 3 AsO 4 Á0.5H 2 O were used as starting reagents in approximate volume ratios of Rb:Ga:As of 1:1:3 for both synthesis batches. For RbGa(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 1.5. The reaction product was washed thoroughly with distilled water, filtered and dried at room temperature. RbGa(HAsO 4 ) 2 formed colourless pseudohexagonal platelets (Fig. 3) and is stable in air. For RbGa 2 As(HAsO 4 ) 6 , which contains As in both tetrahedral and octahedral coordination, no additional water was added and arsenic acid was present in excess to promote the growth of crystals from a melt or even vapour of arsenic acid under extremely acidic conditions. RbGa 2 As(HAsO 4 ) 6 formed large colourless pseudo-octahedral crystals accompanied by small colourless twinned crystals of RbH 3 As 4 O 12 (Schwendtner & Kolitsch, 2007). The crystals of RbGa 2 As-(HAsO 4 ) 6 were extracted mechanically and not further washed; they are hygroscopic and decompose slowly over a period of several years to an amorphous gel and a new, strongly protonated diarsenate containing Rb and Ga (P321, publication in preparation). This slow partial alteration is illustrated in an X-ray powder diffraction pattern (Fig. 5).

Figure 5
Graph of the Rietveld refinement (TOPAS; Bruker, 2009) of RbGa 2 As(HAsO 4 ) 6 , showing the partial alteration of the pseudo-octahedral crystals after an 11-year storage in air. The crystals were hygroscopic and had partly transformed to an amorphous mass. The presence of the relics of the unaltered primary crystals are still visible (pink curve), but a newly crystallized overgrowth of extremely fine fibrous crystals could be attributed to a new strongly protonated Rb-Ga diarsenate with space group P321 (dark-red curve), which will be the subject of a future publication.  (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 RbGa2AsHAsO46.

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.