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 di amond anvil cell and a synchrotron X-ray source in the energy dispersi ve mode. The X-ray data show that three pressure-induced phase transit ions occur in this range in the following sequence: from an orthorhomb ic phase I (Z = 8) to a tetragonal phase II (Z = 1) at 8 GPa; from a t etragonal phase II to an orthorhombic phase III (Z = 4) near 15 GPa, a nd from an orthorhombic phase III to phase IV with an undetermined str ucture near 45 GPa. The volume changes at these transitions are insign ificantly small. Fits to pressure-volume data up to 8 GPa using the Bi rch-Murnaghan equation of state and K-0' = 4 yields K-0 as 142 (12) GP a for orthorhombic PbHfO3 (I); however, a similar fit over the range t o 31 GPa yields 169 (6) GPa for PbHfO3 I, II and III all taken togethe r and assuming K-0' = 4 again. Fitting for PbHfO3 III yields 188 GPa f or the bulk modulus K-0 obtained using a finite strain analysis method recently developed for high pressure phases. The results are in agree ment with the pressure-induced phase transitions observed in a high pr essure Raman study up to 40 GPa.