Pressure-Induced Structural Transition Trends in Nanocrystalline Rare-Earth Sesquioxides: A Raman Investigation

The article presents a concise review of our investigations on the preferred transition paths followed as well as high-pressure-induced structural changes in nanocrystalline rare-earth sesquioxides that include Y2O3, Sm2O3,Gd2O3, Eu2O3, Dy2O3, Ho2O3, and Yb2O3. The starting phase in all samples was predominantly cubic, as characterized using X-ray diffraction and Raman spectroscopy. The pressure-induced structural changes were primarily tracked via changes in phonon modes in the Raman spectra. The structural transition sequences demonstrated behaviors differing from the polycrystalline trends usually observed; however, it was interesting to note that the onset pressures and subsequent shifts in the phonon modes followed a trend similar to the unpressurized samples when observed with respect to the f-electron number. The mode Gruneisen parameters were estimated from the high-pressure data, which indicated a swifter response to external stimuli as the particle size reduced below an average of 50 nm. It was also inferred that the presence of traces of clisordered/nonstoichiometric material directly affected the structural transition sequence. A summary of the results and the mechanisms leading to such structural transitions is discussed.