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

The isostructural compounds caesium deca­sodium tetra­zirconium nona­arsenate, CsNa(10)Zr(4)(AsO(4))(9), and caesium deca­sodium tetra­hafnium nona­arsenate, 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 MO(6) octa­hedra and AsO(4) tetra­hedra 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 octa­hedral geometry and the coordination polyhedra of the partially occupied sodium ions are variously trigonal bipyramidal, tetra­hedral, square pyramidal and trigonal pyramidal. Site symmetries: Cs [Image: see text] ; 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.