Syntheses and crystal structures of new vanadium(IV) oxyphosphates M(VO) 2(PO 4) 2 with M = Co, Ni

We have extended our research interest on titanium oxyphosphates (M II(TiO) 2(PO 4) 2, with M II = Mg, Fe, Co, Ni, Cu, Zn) to vanadium oxyphosphates M II(V IVO) 2(PO 4) 2 (M II = Co, Ni). For each compound two phases, named α and β according to synthesis conditions, have been stabilized at room temperature, then characterized. The four crystal structures M(VO) 2(PO 4) 2 (α and β for M = Co, Ni) have been determined in monoclinic P2 1/ c space group using X-ray single crystals diffraction data. Structure of the α phase is derived from the Li(TiO)(PO 4) (orthorhombic Pnma) and LiNi 0.50(TiO) 2(PO 4) 2 (monoclinic P2 1/ c) types, with cell parameters: a = 6.310(1) Å, b = 7.273(1) Å, c = 7.432(1) Å, β = 90.43(1)° for M = Co, and a = 6.297(2) Å, b = 7.230(2) Å, c = 7.421(2) Å, β = 90.36(2)° for M = Ni. Structure of the β phase is derived from the Ni(TiO) 2(PO 4) 2-type (monoclinic P2 1/ c) with cell parameters: a = 7.2742(2) Å, b = 7.2802(2) Å, c = 7.4550(2) Å, β = 120.171(2)° for M = Co, and a = 7.2691(2) Å, b = 7.2366(2) Å, c = 7.4453(2) Å, β = 120.231(2)° for M = Ni. All these structures consist of a three dimensional (3D) framework built up of infinite chains of tilted corner-sharing [VO 6] octahedra, cross-linked by corner-sharing [PO 4] tetrahedra. The M 2+ ion (M = Co, Ni) is located in a triangular based antiprism which shares faces with two [VO 6] octahedra. Structural filiation is discussed based on a common structural unit, a sheet where divalent cations M 2+ (M = Co, Ni) are inserted. A thermal study of the α ↔ β transition is also presented. [Display omitted]