Abstract
The x-ray-diffraction study of nanosized ${\mathrm{CeO}}_{2}$ was carried to pressures of 38.6 GPa using an energy dispersive synchrotron-radiation technique in a diamond-anvil cell. At a pressure of 22.3 GPa, nano-${\mathrm{CeO}}_{2}$ starts to transform to an orthorhombic $\ensuremath{\alpha}\ensuremath{-}{\mathrm{PbCl}}_{2}$ structure. This pressure is significantly lower than the transition pressure of 31 GPa for phase transformation in the bulk ${\mathrm{CeO}}_{2}.$ The high-pressure phase is unquenchable and distorts to a hexagonal structure upon release of pressure to ambient conditions. The nanosized cubic fluorite phase has a bulk modulus ${(B}_{0})$ of $328\ifmmode\pm\else\textpm\fi{}12$ GPa, much higher than that of the macrosize ${\mathrm{CeO}}_{2}$ with a ${B}_{0}$ of 230 GPa. There is a large volume decrease of 9.4% in phase transformation from the fluorite to $\ensuremath{\alpha}\ensuremath{-}{\mathrm{PbCl}}_{2}$ structure. Such a phase transformation may occur via a large volume collapse and an unstable high-pressure phase causing a reduction of transition pressure in this type of nanomaterial.
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