THE HIGH-PRESSURE PHASE-TRANSITION SEQUENCE FROM THE RUTILE-TYPE THROUGH TO THE COTUNNITE-TYPE STRUCTURE IN PBO2

Four phase transitions were observed in PbO2 under pressure up to 47 G Pa using x-ray diffraction in a diamond anvil cell. At close to 4 GPa, rutile-structured PbO2 underwent a second-order transition to an orth orhombic, CaCl2-type phase. Above 7 GPa this CaCl2-type phase transfor med to a cubic phase with a Pa (3) over bar modified fluorite structur e. A transition to an orthorhombic phase was observed at 11.4 GPa. The intensities of the diffraction lines in this orthorhombic phase indic ate a displacement of the lead ions from the fee positions occupied in the cubic phase. This orthorhombic phase has similar cell constants ( a = 10.027(2) Angstrom, b = 5.246(1) Angstrom, and c = 5.116(1) Angstr om at 26 GPa) to the orthorhombic I phase of ZrO2 and also that of HfO 2, and could be isostructural. A further transition began above 29 GPa to a cotunnite-type phase, with space group Pnam, and Z = 4, and with a = 5.443(18) Angstrom, b = 6.346(17) Angstrom, and c = 3.368(8) Angs trom at 47 GPa. The coordination number of the lead ion is 6 in the fi rst two phases, 6 + 2 in phase Ill, most probably 7 in phase IV and 9 in phase V. The volume decreases observed in the three first-order tra nsitions are 6.9, 1.4, and 7.5% at 7, 11.4, and 29 GPa, respectively. The two higher-pressure transitions were reversible, whereas the cubic phase transformed to alpha-PbO2 upon decompression, and this was reta ined down to ambient pressure. This is the first time a cotunnite-type structure has been adopted by a group IVb dioxide, which has implicat ions for the high-pressure behaviour of the homologous compounds with smaller cations: SiO2, GeO2 and SnO2.