Abstract

The crystal structure of the new mixed oxalate Cd₂Zr(C₂O₄)₄·(4 + n)H₂O has been solved from single-crystal diffraction data. A powder diffraction study has shown that the compound crystallises with orthorhombic symmetry. The structure has been solved in space group Imm2 (Z = 2). The structure is built from chains of eight-fold coordinated cadmium atoms linked to eight-fold coordinated zirconium atoms through bidentate oxalate groups. The structure has an open framework exhibiting tunnels with square and ellipsoidal cross-sections, filled with ‘zeolitic’ water molecules. The initial dehydration process is reversible and occurs with an anisotropic elastic behaviour of the structure framework. The enthalpy and entropy of the dehydration reaction have been determined. They are Δ_rH = 49(2) kJ mol⁻¹ and Δ_rS = 123(3) J K⁻¹ mol⁻¹ for the initial dehydration stage, which is related to the departure of ‘zeolitic’ water molecules, and Δ_rH = 80(1) kJ mol⁻¹and Δ_rS = 141(2) J K⁻¹ mol⁻¹ for the second stage related to the departure of bonded water molecules. The complete decomposition scheme into cubic zirconium oxide and cadmium oxide is reported from a thermal analysis study, including temperature-dependent powder diffraction.