Unraveling microstrain-promoted structural evolution and thermally driven phase transition in c − Sc 2 O 3 nanocrystals at high pressure

Here, we report an irreversible cubic-to-monoclinic structural transition in cubic $c\text{\ensuremath{-}}{\mathrm{Sc}}_{2}{\mathrm{O}}_{3}$ nanocrystals which occur at pressures above $\ensuremath{\sim}8.9\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ upon nonhydrostatic compression in association with a pronounced volume collapse. This phase-transition--induced anomaly is further confirmed by our experimental Raman spectroscopy measurements and theoretical predictions. After annealing, however, this high-pressure monoclinic $m\text{\ensuremath{-}}{\mathrm{Sc}}_{2}{\mathrm{O}}_{3}$ phase undergoes a reversible back-transformation to the cubic counterpart at $\ensuremath{\sim}1123\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and 9.0 GPa. Our observed transition pressure of $\ensuremath{\sim}8.9\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ for the cubic-to-monoclinic structural evolution is significantly lower than that from the previously diamond-anvil-cell--based hydrostatic x-ray experiments because of the existence of internal microscopic stress and/or high-stress concentration in the specimen caused by grain-to-grain contacts upon nonhydrostatic compression, which promoted the cubic-to-monoclinic structural transition. Moreover, we have reported new thermoelastic properties of $c\text{\ensuremath{-}}{\mathrm{Sc}}_{2}{\mathrm{O}}_{3}$ nanocrystals at simultaneous high-pressure and high-temperature conditions. These findings/results may have significant implications for the design of phase-switching devices and for the exploration of the structural relationship among sesquioxides for their uses in extreme environments.