Crystal structure, phase transition and anisotropic thermal expansion of barium zirconium diorthophosphate, BaZr(PO 4) 2

Abstract

The crystal structure of BaZr(PO 4) 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 , β = 93.086 ( 1 ) ° 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(SO 4) 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.