NaSr(AsO4)(H2O)9: the (Sr,As) analogue of nabaphite and nastrophite

The crystal structure of the title compound, sodium strontium orthoarsenate(V) nonahydrate, is isotypic with NaSr(PO4)(H2O)9 and the minerals nabaphite [NaBa(PO4)(H2O)9] and nastrophite [Na(Sr,Ba)(PO4)(H2O)9]. The Na and Sr atoms are located on threefold rotation axes and are in the centres of slightly distorted Na(H2O)6 octahedra and Sr(H2O)9 tricapped trigonal prisms, respectively. A framework structure is established via edge-sharing of these polyhedra. Disordered AsO4 tetrahedra (with threefold symmetry) are situated in the interstitial space of the framework. Although reasonable H-atom positions of the water molecules were not established, close O⋯O contacts between the disordered AsO4 tetrahedra and the water molecules suggest strong O—H⋯O hydrogen bonding.

The crystal structure of the title compound, sodium strontium orthoarsenate(V) nonahydrate, is isotypic with NaSr(PO 4 )-(H 2 O) 9 and the minerals nabaphite [NaBa (PO 4 )(H 2 O) 9 ] and nastrophite [Na(Sr,Ba)(PO 4 )(H 2 O) 9 ]. The Na and Sr atoms are located on threefold rotation axes and are in the centres of slightly distorted Na(H 2 O) 6 octahedra and Sr(H 2 O) 9 tricapped trigonal prisms, respectively. A framework structure is established via edge-sharing of these polyhedra. Disordered AsO 4 tetrahedra (with threefold symmetry) are situated in the interstitial space of the framework. Although reasonable Hatom positions of the water molecules were not established, close OÁ Á ÁO contacts between the disordered AsO 4 tetrahedra and the water molecules suggest strong O-HÁ Á ÁO hydrogen bonding.
The Na(H 2 O) 6 octahedron is slightly distorted. The corresponding Na-O bond lenghts are in the usual range with an average of 2.40 Å, conform with the values in the isotypic compounds (NaSr(PO 4 )(H 2 O) 9 : 2.41 Å; nabaphite: 2.42 Å; nastrophite: 2.42 Å) and the sum of the ionic radii of 2.41 Å, as calculated for six-coordinated Na (Shannon, 1976).
The Sr 2+ ion is surrounded by 9 oxygen atoms with an average Sr-O distance of 2.663 Å, in good agreement with the phosphate analogue (2.668 Å; Takagi et al., 1982) and the sum of the ionic radii of 2.67 Å, as calculated for nine-coordinated Sr (Shannon, 1976).
The environment of the disordered AsO 4 group consists of 16 water molecules with donor (D) -acceptor (A) distances between 2.5 and 3.0 Å (see Table 2). The overcrowding of water molecules (only 12 surrounding O atoms are expected, considering an ordered tetrahedral configuration for the arsenate unit with three donator atoms) may be the reason for the disorder of the AsO 4 group. The average As-O bond length for the disordered AsO 4 group is 1.685 Å, a value in very good agreement with those of 1.682 Å (Baur, 1981) and 1.686 Å (Schwendtner, 2008).

Experimental
Crystals of the title compound were obtained during phase formation studies in the system Sr-As-O (Weil et al., 2009) from hydrous solutions. All chemicals used were of analytical grade and employed without further purification. To a saturated Sr(OH) 2 . 8H 2 O solution a diluted arsenic acid solution was dropwise added which resulted in a flocculent white precipitate (pH ca. 9). A concentrated NaOH solution was then added until a pH of 12 was reached. The resulting suspension was boiled for an hour. Then the precipitate was filtered off and the remaining solution was left to stand for several days.
Besides few crystals of SrHAsO 4 (Mihajlovic & Effenberger, 2006), colourless columnar crystals of the title compound up to several mm in length were obtained after complete evaporation of water.

Refinement
The oxygen atoms O4 and O6 of the arsenate group were clearly discernible from Fourier maps. Consideration of full occupancy of these sites resulted in high electron densities of ca 4 e Å 3 at a distance of ca 1.7 Å from As, indicating other disordered oxygen atoms. Therefore four additional O atoms were considered in the final model. Free refinement of the site occupancy factors (s.o.f.) of all six O atoms attached to arsenic resulted in a composition very close to the theoretical value.
In the last refinement cycles the s.o.f.'s were constrained to meet the criterion for electroneutrality. The six disordered O atoms were finally refined with isotropic displacement parameters. No reasonable positions of the H atoms attached to the water molecules (O1, O2, O3) could be found in difference Fourier maps which may be due to the disorder of the AsO 4 tetrahedron and the resulting complex hydrogen bonding scheme. Therefore all H atoms were excluded from the refinement.
The highest remaining peak in the final difference Fourier map is 0.49 Å from As and the deepest hole is 0.43 Å from the same atom. Fig. 1

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 )
x y z U iso */U eq Occ. (