A sodium calcium arsenate, NaCa(AsO4)

Abstract

The title compound, NaCa(AsO4), was synthesized using a hydrothermal method at 633-643 K. It has a dense structure composed of alternating layers of distorted [CaO6] octa-hedra and layers of [AsO4] tetra-hedra and distorted [NaO6] octa-hedra, stacked along the a axis. The As, Ca and two O atoms lie on the mirror plane at y = 1/4 (i.e. 4c), while the Na atom lies on an inversion centre (1/2, 1/2, 0) (i.e. 4b). Each distorted [CaO6] octa-hedron shares four equatorial common O vertices with four neighboring octa-hedra, forming a layer parallel to (100), whereas each distorted [NaO6] octa-hedron shares two opposite edges with two neighboring ones, forming a chain running along [010]. Each isolated [AsO4] tetra-hedron shares two edges with two different [NaO 6] octa-hedra in one [NaO6] chain and a vertex with another chain. Simultaneously the above [AsO4] tetra-hedron located in a four-membered [CaO6] ring shares one edge of its base facet with one [CaO6] octa-hedron and three corners with three other [CaO 6] octa-hedra of one [CaO6] layer, and the remaining apex is shared with another [CaO6] layer. [NaO6] octa-hedra and [CaO6] octa-hedra are linked to each other by sharing edges and vertices.