Crystal structures and phase transitions of SrZr(PO 4) 2–BaZr(PO 4) 2 solid solutions

Abstract

A complete series of solid solutions was prepared in the SrZr(PO 4) 2–BaZr(PO 4) 2 system and examined by conventional X-ray powder diffraction (XRPD). The crystals of Sr x Ba 1− x Zr(PO 4) 2 with x⩽0.1 were isomorphous with yavapaiite (KFe(SO 4) 2, space group C2/ m). The solid solution with 0.2⩽ x⩽0.7 has been composed of a new phase, showing a superstructure along the a-axis ( c-axis of the yavapaiite substructure). The crystals with 0.8⩽ x⩽0.9 were composed of both the new phase and the triclinic phase, the latter being isostructural with SrZr(PO 4) 2 ( x=1). The crystal structure of the new phase has been determined using direct methods, and it has been further refined by the Rietveld method. The crystal of Sr 0.7Ba 0.3Zr(PO 4) 2 ( x=0.7) is monoclinic (space group P2/ c, Z=4 and D x /Mg m −3=3.73) with a=1.53370(8) nm, b=0.52991(3) nm, c=0.84132(4) nm, β=92.278(1)° and V=0.68321(6) nm 3. Final reliability indices are R wp=7.32%, R p=5.60% and R B=3.22%. The powder specimen was also examined by high-temperature XRPD and differential thermal analysis (DTA) to reveal the occurrence of two phase transitions during heating; the space group changed from P2/ c to C2/ m at ∼400 K, followed by the monoclinic-to-hexagonal (or trigonal) transition at 1060 K. The P2/ c-to- C2/ m transition has been, for the first time, described in the yavapaiite-type compounds.