In situx-ray diffraction study of the pressure-induced phase transformation in nanocrystallineCeO2

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 $\alpha -{\mathrm{PbCl}}_2$ 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 $328\pm 12$ 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 $\alpha -{\mathrm{PbCl}}_2$ 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.