Post-Rh2O3(II)transition and the high pressure-temperature phase diagram of gallia: A first-principles and x-ray diffraction study

Abstract

The high pressure-temperature behavior of gallium sesquioxide (gallia) has been investigated by combining first-principles density functional computations and high-pressure x-ray diffraction measurements up to $2\phantom{\rule{0.3em}{0ex}}\mathrm{Mbar}$. Calculations show that at $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the transformation from corundum-type $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ to ${\mathrm{Rh}}_{2}{\mathrm{O}}_{3}(II)$-type structure at $32\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ is followed by second one to the $Cmcm$ structure at $141\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. This post-${\mathrm{Rh}}_{2}{\mathrm{O}}_{3}(II)$ transition was successfully confirmed experimentally at $164\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and $1300\ifmmode\pm\else\textpm\fi{}500\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Quasiharmonic free energy calculations show that both transformations display negative Clapeyron slope. Structural sequences, transition pressures, and Clapeyron slopes are found well comparable in gallia and alumina. Results are suggestive that the high-pressure stability of the $Cmcm$ structure is a general property even in group IIIB sesquioxides in the pressure range beyond $100\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, though the structure has cation sites with two distinct oxygen coordination numbers.