Equation of state of gallium oxide to70GPa: Comparison of quasihydrostatic and nonhydrostatic compression

Synchrotron x-ray diffraction and diamond-anvil cell techniques were used to characterize pressure induced structural modifications in gallium oxide. Gallium oxide was studied on compression up to $70\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and on the following decompression. The effect of the pressure-transmitting medium on the structural transformations was investigated in two sets of compression and decompression runs, one with nitrogen as a quasihydrostatic pressure-transmitting medium and the other in nonhydrostatic pressure conditions. The x-ray diffraction data showed gradual phase transition from a low-density, monoclinic $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ to a high-density, rhombohedral $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$. With the use of nitrogen as a pressure transmitting medium, the $\ensuremath{\beta}$- to $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ transition begins at about $6.5--7\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and extends up to $\ensuremath{\sim}40\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, confirming recent theoretical calculations. This pressure-driven transition is irreversible and the material decompressed from $70\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ to ambient conditions was composed, in both sets of experimental runs, of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ only. A Birch-Murnaghan fit of the unit cell volume as a function of pressure yielded a zero pressure bulk modulus ${K}_{0}=199(6)\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, and its pressure derivative ${K}_{0}^{\ensuremath{'}}=3.1(4)$ for the$\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ phase, and ${K}_{0}=220(9)\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and ${K}_{0}^{\ensuremath{'}}=5.9(6)$ for the $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ phase for the experiments performed in quasihydrostatic compression conditions. When for the same experiment ${K}_{0}^{\ensuremath{'}}$ is held at 4, then the bulk modulus values are 184(3) and $252(14)\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ for $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ and the $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$, respectively. We compare the results of this work with our previous studies on the high-pressure behavior of nanocrystalline gallium oxide.