Synthesis, structure determination from powder diffraction data, and thermal behavior of lead zirconium oaxalate hydrate Pb2Zr(C2O4)4•nH2O (3 < n < 9)

A new mixed lead zirconium oxalate, Pb2Zr(C2O4)(4). nH(2)O (3 < n < 9), has been prepared from precipitation of a nitric solution of lead and zirconium cations by an oxalic solution at 345 K. The crystal structure of Pb2Zr(C2O4)(4). 6H(2)O has been determined ab initio from powder diffraction data collected with conventional monochromatic X-rays. The symmetry is monoclinic, space group C2/c (No. 15), cell dimensions a = 9.537(1) Angstrom, b = 29.622(3) Angstrom, c = 8.9398(9) Angstrom, beta = 121.19(4)degrees, and Z = 4. The complex three-dimensional structure is built from 8-fold-coordinated zirconium atoms and 9-fold- and 8-fold-coordinated lead atoms linked by oxalate groups, Only one water molecule has been found free in the structure. The dehydration is reversible, and the cell parameters vary between the composition limits without a major change of the crystal structure framework. A thermodynamical study has demonstrated the zeolitic nature of the water molecules. The enthalpy, Delta(r)H, and entropy, Delta(r)S, of the dehydration reaction have been determined as a function of the number of zeolitic water molecules. The complete decomposition scheme of the anhydrous phase Pb2Zr(C2O4)(4) into PbZrO3 and finally ZrO2, studied from temperature-dependent X-ray powder diffraction and thermogravimetric analysis, is carefully described.