An in-situ high pressure x-ray diffraction study of PbHfO₃ to 52.5 GPa at room temperature and pressure-induced phase transformations

In-situ high pressure X-ray diffraction studies have been carried out on lead hafnate PbHfO3 up to 52.5 GPa at room temperature using the diamond anvil cell and a synchrotron X-ray source in the energy dispersive mode. The X-ray data show that three pressure-induced phase transitions occur in this range in the following sequence: from an orthorhombic phase I (Z = 8) to a tetragonal phase II (Z = 1) at 8 GPa; from a tetragonal phase II to an orthorhombic phase III (Z = 4) near 15 GPa, and from an orthorhombic phase III to phase IV with an undetermined structure near 45 GPa. The volume changes at these transitions are insignificantly small. Fits to pressure-volume data up to 8 GPa using the Birch-Murnaghan equation of state and K′0 = 4 yields K0 as 142 (12) GPa for orthorhombic PbHfO3 (I); however, a similar fit over the range to 31 GPa yields 169 (6) GPa for PbHfO3 I, II and III all taken together and assuming K′0 = 4 again. Fitting for PbHfO3 III yields 188 GPa for the bulk modulus K0 obtained using a finite strain analysis method recently developed for high pressure phases. The results are in agreement with the pressure-induced phase transitions observed in a high pressure Raman study up to 40 GPa