Abstract
Four phase transitions were observed in 
under pressure up to 47 GPa using x-ray diffraction in a diamond anvil cell. At close to 4 GPa, rutile-structured underwent a second-order transition to an orthorhombic, 
-type phase. Above 7 GPa, this -type phase transformed to a cubic phase with a 
modified fluorite structure. A transition to an orthorhombic phase was observed at 11.4 GPa. The intensities of the diffraction lines in this orthorhombic phase indicate a displacement of the lead ions from the fcc positions occupied in the cubic phase. This orthorhombic phase has similar cell constants (a = 10.027(2) Å, b = 5.246(1) Å, and c = 5.116(1) Å at 26 GPa) to the orthorhombic I phase of 
and also that of 
, 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) Å, b = 6.346(17) Å, and c = 3.368(8) Å at 47 GPa. The coordination number of the lead ion is 6 in the first two phases, 6 + 2 in phase III, most probably 7 in phase IV and 9 in phase V. The volume decreases observed in the three first-order transitions 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 
upon decompression, and this was retained down to ambient pressure. This is the first time a cotunnite-type structure has been adopted by a group IVb dioxide, which has implications for the high-pressure behaviour of the homologous compounds with smaller cations: 
and 
.
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