M₂VOP₂O₇ (M = Rb, Cs): Two vanadyl pyrophosphates with a layer structure

The new compounds Rb₂VOP₂O₇ and Cs₂VOP₂O₇ were prepared by heating stoichiometric mixtures of Rb₃VO₄ (or Cs₃VO₄), VO₂, V, and P₂O₅ in sealed quartz tubes. For Rb₂VOP₂O₇, orthorhombic, P2₁2₁2₁, a = 7.101(1), b = 9.174(2), c = 12.801(4) Å, V = 834.0(4)° ų, Z = 4, and R = 5.56%, R_w = 5.88% for 928 independent reflections. For Cs₂VOP₂O₇, orthorhombic, Pnma, a = 13.280(3), b = 7.247(2), c = 9.518(2) Å, V = 916.0(4) ų, Z = 4, and R = 4.12%, R_w = 4.53% for 1118 independent reflections. Both structures consist of corrugated layers of vanadyl pyrophosphate with the alkali metal cations between the layers. Each layer is built up from corner-sharing VO₅ square pyramids and P₂O₇ groups. These two compounds adopt a markedly different structure from K₂VOP₂O₇.