Flux syntheses and single-crystal structures of CsNa10M4(AsO4)9 (M = Zr, Hf)

The isostructural compounds caesium decasodium tetrazirconium nonaarsenate, CsNa10Zr4(AsO4)(9), and caesium decasodium tetrahafnium nonaarsenate, CsNa10Hf4(AsO4)(9), arose as unexpected single-crystal products from the reactions of Na2CO3, MO2 (M = Zr, Hf) and As2O5 in a eutectic flux of NaCl and CsCl. They consist of MO6 octahedra and AsO4 tetrahedra sharing vertices to generate three-dimensional polyhedral networks encapsulating the caesium and sodium ions. The MO6 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 (3) over bar; 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.