Crystal structures and phase transitions of SrZr(PO4)2–BaZr(PO4)2 solid solutions

A complete series of solid solutions was prepared in the SrZr(PO4)2–BaZr(PO4)2 system and examined by conventional X-ray powder diffraction (XRPD). The crystals of SrxBa1−xZr(PO4)2 with x⩽0.1 were isomorphous with yavapaiite (KFe(SO4)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(PO4)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 Sr0.7Ba0.3Zr(PO4)2 (x=0.7) is monoclinic (space group P2/c, Z=4 and Dx/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) nm3. Final reliability indices are Rwp=7.32%, Rp=5.60% and RB=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.

Crystal structure of Sr0.7Ba0.3Zr(PO4)2.Figure optionsDownload as PowerPoint slide