Crystal structure, phase transition and anisotropic thermal expansion of barium zirconium diorthophosphate, BaZr(PO4)2

The crystal structure of BaZr(PO4)(2) at 298 K was determined from conventional X-ray powder diffraction data using direct methods, and it was further refined by the Rietveld method. The structure was monoclinic (space group C2/m, Z = 2) with a = 0.85629(3) nm, b = 0.53082(2) nm, c = 0.78956(2) nm, beta = 93.086(1)degrees and V = 0.35836(2) nm(3). Final reliability indices were R-wp = 8.21%, R-p = 5.64% and R-B = 2.92%. The atom arrangement is similar to that of yavapaiite (KFe(SO4)(2)), however, these crystal structures differ distinctly in the coordination numbers of barium and potassium atoms; the former is tenfold coordinated, whereas the latter is sixfold coordinated. The powder specimens were also examined by high-temperature XRD and DTA to reveal the occurrence of a phase transition from monoclinic to orthorhombic at 732 K during heating. Upon cooling the reverse transition occurred at 710 K. The monoclinic crystal expanded almost one-dimensionally along [503] during the heating process. The orthorhombic phase also showed a tendency to expand one-dimensionally along the c-axis above 732 K. (c) 2005 Elsevier Inc. All rights reserved.