In situ high-pressure synchrotron x-ray diffraction study of Zr2(WO4)(PO4)2 up to 16 GPa

The negative thermal-expansion material Zr-2(WO4)(PO4)(2) adopts the orthorhombic Sc-2(WO4)(3) structure under ambient conditions. Synchrotron x-ray powder diffraction was used to study its behavior on compression in a diamond-anvil cell up to 16 GPa. Three crystalline-to-crystalline phase transitions were observed. The material was orthorhombic (Pnca) in the pressure range 0.0-1.37 GPa, monoclinic between 1.68 and 3.7 GPa, monoclinic with a different structure between 3.7 and 6.3 GPa, and triclinic between 7.4 and 14 GPa. Bulk moduli for these phases were estimated using a Birch-Murnaghan equation of state to be 49(2), 17(1), 37(1), and 76(7) GPa, respectively. The first two phase transitions were reversible on decompression. Irreversible partial amorphization was observed above 14 GPa. This sequence of phase transitions, and the pressure at which the first transition occurs, is significantly different from that previously observed for A(2)(MO4)(3) (A is any +3 ion, M is Mo or W) compounds that adopt a Sc-2(WO4)(3) structure under ambient conditions.