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=2Z=2) with a=0.85629(3)nm, b=0.53082(2)nm, c=0.78956(2)nm, β=93.086(1)°β=93.086(1)° and V=0.35836(2)nm3. Final reliability indices were Rwp=8.21%Rwp=8.21%, Rp=5.64%Rp=5.64% and RB=2.92%RB=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.

Graphical abstractPart of the monoclinic BaZr(PO4)2 structure viewed along [12¯0].Figure optionsDownload full-size imageDownload as PowerPoint slide