Flux syntheses and single-crystal structures of CsNa (AsO) ( = Zr, Hf).

The isostructural compounds caesium deca-sodium tetra-zirconium nona-arsenate, CsNaZr(AsO), and caesium deca-sodium tetra-hafnium nona-arsenate, CsNaHf(AsO), arose as unexpected single-crystal products from the reactions of NaCO, MO ( = Zr, Hf) and AsO in a eutectic flux of NaCl and CsCl. They consist of O octa-hedra and AsO tetra-hedra sharing vertices to generate three-dimensional polyhedral networks encapsulating the caesium and sodium ions. The MO groups share all their vertices with adjacent As atoms but the As atoms have one or two 'terminal' O atoms not bonded to Zr or Hf. The Cs ion adopts a squashed octa-hedral geometry and the coordination polyhedra of the partially occupied sodium ions are variously trigonal bipyramidal, tetra-hedral, square pyramidal and trigonal pyramidal. Site symmetries: Cs ; 3; As 1 and 2; O 1; Na 1, 2 and 3. The = Zr crystal was refined as an obverse/reverse rhombohedral twin.