Redetermination of the hexagonal struvite analogue Cs[Mg(OH2)6](PO4)

The structure of the hexagonal modification of caesium hexaaquamagnesium phosphate has been redetermined from single-crystal X-ray data. The previous refinement from photographic data [Ferrari, Calvaca & Nardelli (1955 ▶). Gazz. Chim. Ital. 85, 1232–1238] was basically confirmed, but with all H atoms located and with all non H-atoms refined with anisotropic displacement parameters. The structure can be derived from the NiAs structure type: the PO4 tetrahedra (3m. symmetry) are on the Ni positions and the complex [Mg(OH2)6] octahedra (3m. symmetry) are on the As positions. The building units are connected to each other by hydrogen bonds. The Cs+ cations (3m. symmetry) are located in the voids of this arrangement and exhibit a distorted cuboctahedral 12-coordination by the O atoms of the water molecules.

The structure of the hexagonal modification of caesium hexaaquamagnesium phosphate has been redetermined from single-crystal X-ray data. The previous refinement from photographic data [Ferrari, Calvaca & Nardelli (1955). Gazz. Chim. Ital. 85, 1232-1238 was basically confirmed, but with all H atoms located and with all non H-atoms refined with anisotropic displacement parameters. The structure can be derived from the NiAs structure type: the PO 4 tetrahedra (3m. symmetry) are on the Ni positions and the complex [Mg(OH 2 ) 6 ] octahedra (3m. symmetry) are on the As positions. The building units are connected to each other by hydrogen bonds. The Cs + cations (3m. symmetry) are located in the voids of this arrangement and exhibit a distorted cuboctahedral 12-coordination by the O atoms of the water molecules.

Comment
Numerous compounds with general formula A[B(OH 2 ) 6 ]XO 4 , where A = alkali metal, NH 4 or Tl, B = Mg or first row transition 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 & Jeffery, 1979a,b). However, for the isoformular compounds Cs[Mg(OH 2 ) 6 ](XO 4 ), where X = P and As, cubic and hexagonal forms were reported and structurally characterized (Ferrari et al., 1955;Massa et al., 2003). The cubic polymorph forms under hydrothermal conditions, whereas the hexagonal form is obtained under normal pressure and temperatures. All these structures can be described in terms of closed-packed layers with different stacking sequences (Massa et al., 2003).
In this communication the redetermination of hexagonal Cs[Mg(OH 2 ) 6 ](PO 4 ) is reported. The previous refinement from photographic data (Ferrari et al., 1955) was basically confirmed, but with all H atoms located and with all non H-atoms refined with anisotropic displacement parameters.
In addition to the description of the hexagonal Cs[Mg(OH 2 ) 6 ](PO 4 ) structure in terms of closed-packed layers (Massa et al., 2003), the structure can be described as a derivative of the NiAs structure type. The centres of the slightly distorted PO 4 tetrahedra (3m. symmetry) are situated on the Ni positions, whereas the centres of the likewise slightly distorted complex  (Table 1). These building units are linked via medium-strong hydrogen bonds (Table 2, Fig. 2). Details and differences of the hydrogen bonding schemes in cubic, hexagonal and struvite-type structures were discussed by Massa et al. (2003). The Cs + cations are located in the voids of this arrangement and exhibit a distorted cuboctahedral 12-coordination [9+3] to the oxygen atoms of the water molecules (Table 1).

Experimental
Colourless crystals of Cs[Mg(OH 2 ) 6 ](PO 4 ) with an edge-length up to 2 mm and mostly spherical habit 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 PO 4 (50 ml) was carefully poured over the gel. This solution was then adjusted to pH 8.5 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.

supplementary materials sup-2 Refinement
The positions of the H atoms were found from difference Fourier maps and were refined with soft distance restraints (d(O-H) = 0.90 (5) Å) and a common U iso parameter. Fig. 1  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.

Figures
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )