Olivine-type NaCd(AsO4)

The title compound, sodium cadmium orthoarsenate, adopts the olivine [Mg2(SiO4)] structure type in space group Pnma, with Na (site symmetry -1) and Cd (.m.) replacing the two Mg positions, and the AsO4 tetrahedron (.m.) the SiO4 tetrahedron. The crystal structure is made up of a nearly hexagonal closed-packed arrangement of O atoms stacked along [001]. The Na and Cd atoms occupy one half of the octahedral voids in alternate layers stacked along [100], and one eighth of the tetrahedral voids are occupied by As atoms.

NaCd(AsO 4 ) crystallizes in the olivine [Mg 2 (SiO 4 )] structure type. A review on the crystal chemistry of olivines and spinels has been published some time ago by Brown (1982). The olivine structure is characterized by an almost hexagonal closed-packed arrangement of oxygen atoms parallel to (001) with a stacking sequence ABAB along [001] with one half of the octahedral voids occupied by magnesium sites, and one-eighth of the tetrahedral voids by silicon sites. In the structure of NaCd(AsO 4 ), the two magnesium sites and the silicon site are replaced by unique sodium and cadmium sites and an arsenic site, respectively. The resulting NaO 6 octahedron has 1 symmetry, and the CdO 6 octahedron and the AsO 4 tetrahedron both have .m. symmetry.
In comparison with the isotypic structure of NaCd(PO 4 ) (Ivanov et al., 1974), the Cd-O and Na-O distances are virtually the same, with mean bond lengths for the CdO 6 and NaO 6 octahedra of 2.307 (71) Å and 2.398 (54) Å in the phosphorus compound, and of 2.314 (75) Å and 2.421 (64) Å, respectively, in the arsenic compound. As expected, only the distances within the PO 4 (mean P-O distance 1.541 (12) Å) and AsO 4 (1.693 (13) Å) tetrahedra differ, because of the different sizes of P(V) and As(V).

Experimental
To an aqueous 0.05 M solution (20 ml) of cadmium acetate, an aqueous solution of 0.05 M Na 2 HAsO 4 (20 ml) was added.
Together with the mother liquor, parts of the colourless precipitate were transferred to a 10 ml Teflon insert (filling degree one-half). The insert was then sealed and heated in a steel autoclave under autogenous pressure at 338 K for ten days.
Few colourless single crystals of the title compound with a block-like form were obtained after the reaction time, and were manually separated from the reaction mixture that was not further examined.

Refinement
Atomic coordinates of the isotypic NaCd(PO 4 ) (Ivanov et al., 1974) in space group Pnma were taken as starting parameters for refinement of NaCd(AsO 4 ). Besides a structure refinement of NaCd(PO 4 ) in Pnma, also refinements of this structure in the non-centrosymmetric space group No. 33 were performed with settings in Pn2 1 a (Ivanov et al., 1974) and P2 1 nb (Hata et al., 1979). However, adopting these non-centrosymmetric structure models for refinement of NaCd(AsO 4 ) converged with significantly higher residual parameters and with large correlation matrix elements. Therefore the structure model in space group Pnma was considered as being correct, in agreement with other olivine-type structures. Structure data were finally standardized with STRUCTURE TIDY (Gelato & Parthé, 1987).

Figure 1
The olivine-type crystal structure of NaCd(AsO 4 ) in a projection along [001]. Displacement ellipsoids are drawn at the 74% probability level. 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 R-factors(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.