Flux syntheses and single-crystal structures of CsNa 10 M 4 (AsO 4 ) 9 ( M = Zr, Hf).

The isostructural compounds caesium deca-sodium tetra-zirconium nona-arsenate, CsNa 10 Zr 4 (AsO 4 ) 9 , and caesium deca-sodium tetra-hafnium nona-arsenate, CsNa 10 Hf 4 (AsO 4 ) 9 , arose as unexpected single-crystal products from the reactions of Na 2 CO 3 , MO 2 ( M = Zr, Hf) and As 2 O 5 in a eutectic flux of NaCl and CsCl. They consist of M O 6 octa-hedra and AsO 4 tetra-hedra sharing vertices to generate three-dimensional polyhedral networks encapsulating the caesium and sodium ions. The MO 6 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 ; M 3; As 1 and 2; O 1; Na 1, 2 and 3. The M = Zr crystal was refined as an obverse/reverse rhombohedral twin.