Redetermination of the cubic struvite analogue Cs[Mg(OH2)6](AsO4)

In contrast to the previous refinement from photographic data [Ferrari et al. (1955 ▶). Gazz. Chim. Ital. 84, 169–174], the present redetermination of the title compound, caesium hexaaquamagnesium arsenate(V), revealed the Cs atom to be on Wyckoff position 4d instead of Wyckoff position 4b of space group F 3m. The structure can be derived from the halite structure. The centres of the complex [Mg(OH2)6] octahedra and the AsO4 tetrahedra (both with 3m symmetry) are on the respective Na and Cl positions. The building units are connected to each other by O—H⋯O hydrogen bonds. The Cs+ cations (3m symmetry) are located in the voids of this arrangement and exhibit a regular cuboctahedral 12-coordination to the O atoms of the water molecules. The O atom bonded to As has 2mm site symmetry (Wyckoff position 24f) and the water-molecule O atom has m site symmetry (Wyckoff position 48h).

In contrast to the previous refinement from photographic data [Ferrari et al. (1955). Gazz. Chim. Ital. 84,[169][170][171][172][173][174], the present redetermination of the title compound, caesium hexaaquamagnesium arsenate(V), revealed the Cs atom to be on Wyckoff position 4d instead of Wyckoff position 4b of space group F43m. The structure can be derived from the halite structure. The centres of the complex [Mg(OH 2 ) 6 ] octahedra and the AsO 4 tetrahedra (both with 43m symmetry) are on the respective Na and Cl positions. The building units are connected to each other by O-HÁ Á ÁO hydrogen bonds. The Cs + cations (43m symmetry) are located in the voids of this arrangement and exhibit a regular cuboctahedral 12coordination to the O atoms of the water molecules. The O atom bonded to As has 2mm site symmetry (Wyckoff position 24f) and the water-molecule O atom has m site symmetry (Wyckoff position 48h).
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: HB2885). ition metal, and X = P or As, are known to crystallize in the orthorhombic struvite (NH 4 [Mg(OH 2 ) 6 ](PO 4 )) structure in space group Pmn2 1 (Whitaker & Jeffrey, 1970a, b). The title compound, (I), Cs[Mg(OH 2 ) 6 ](AsO 4 ), is an isoformular analogue of struvite, but crystallizes in the cubic crystal system. All these structures can be described in terms of closed-packed layers with different stacking sequences (Massa et al., 2003). Phases that are isotypes of struvite, as well as struvite analogues were recently surveyed by Weil (2008). In comparison with the previous refinement from photographic data (Ferrari et al., 1955), the present redetermination of Cs[Mg(OH 2 ) 6 ](AsO 4 ) revealed a different location of the Cs atom, the localization of the H atom and anisotropic displacement parameters for all non H-atoms.

Structure Reports Online
The structure can be described as a derivative of the NaCl structure type (Massa et al., 2003).  (Table   1). These building units are linked via medium-strong hydrogen bonds (Table 2). Details and differences of the hydrogen bonding schemes in cubic, hexagonal and orthorhombic struvite-type structures were discussed in detail by Massa et al. (2003). The Cs + cations are located in the voids of this arrangement and exhibit a regular cuboctahedral 12-coordination to the oxygen atoms of the water molecules (Table 1).

Experimental
Colourless octahedral crystals of Cs[Mg(OH 2 ) 6 ](AsO 4 ) with an edge-length up to 1 mm were grown by means of the gel diffusion technique, following a slightly modified procedure as that given by Banks et al. (1975). Aqueous solutions of 0.025 M MgSO 4 and 0.02 M Na 4 edta (edta = ethylenediaminetetraacetate) were adjusted to pH 10 with NaOH. Commercially available gelatine foils (5 g) were dissolved in the hot resulting 100 ml solution and allowed to form a gel inside a large test tube overnight. When the gel had set, an equivalent amount of a solution of 0.025 M CsH 2 AsO 4 (50 ml) was carefully poured over the gel. This solution was then adjusted to pH 9 with NaOH. The test tube was covered with parafilm and the crystal growth proceeded at the gel-liquid interface and into the gel. Crystals large enough for conventional x-ray analysis grew within one week at room temperature. They were separated mechanically from the gel and were washed with a water/ethanol/acetone (1/3/1) mixture.

Special details
Geometry. All e.s. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )