Anomalous high pressure behaviour in nanosized rare earth sesquioxides

We report Raman spectroscopic studies of the nanosized rare earth sesquioxides, namely yttrium sesquioxide (Y2O3), gadolinium sesquioxide (Gd2O3) and samarium sesquioxide (Sm2O3), under high pressure. The samples were characterized using x-ray diffraction, Raman spectroscopy and atomic force microscopy at atmospheric pressures. Y2O3 and Gd2O3 were found to be cubic at ambient, while Sm2O3 was found to be predominantly cubic with a small fraction of monoclinic phase. The strongest Raman peaks are observed at 379, 344 and 363 cm-1, respectively, for Y2O3, Sm2O3 and Gd2O3. All the samples were found to be nanosized with 50-90 nm particle sizes. The high pressures were generated using a Mao-Bell type diamond anvil cell and a conventional laser Raman spectrometer is used to monitor the pressure-induced changes. Y2O3 seems to undergo a crystalline to partial amorphous transition when pressurized up to about 19 GPa, with traces of hexagonal phase. However, on release of pressure, the hexagonal phase develops into the dominant phase. Gd2O3 is also seen to develop into a mixture of amorphous and hexagonal phases on pressurizing. However, on release of pressure Gd2O3 does not show any change and the transformation is found to be irreversible. On the other hand, Sm2O3 shows a weakening of cubic phase peaks while monoclinic phase peaks gain intensity up to about a pressure of 6.79 GPa. However, thereafter the monoclinic phase peaks also reduce in intensity and mostly disordering sets in which does not show significant reversal as the pressure is released. The results obtained are discussed in detail