Equation of state of gallium oxide to 70 GPa: 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 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 beta- Ga2O3 to a high- density, rhombohedral alpha-Ga2O3. With the use of nitrogen as a pressure transmitting medium, the beta- to alpha- Ga2O3 transition begins at about 6.5 - 7 GPa and extends up to similar to 40 GPa, confirming recent theoretical calculations. This pressure- driven transition is irreversible and the material decompressed from 70 GPa to ambient conditions was composed, in both sets of experimental runs, of alpha- Ga2O3 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 ) GPa, and its pressure derivative K-0(') = 3.1 ( 4 ) for the beta- Ga2O3 phase, and K-0= 220 ( 9 ) GPa and K-0'= 5.9 ( 6 ) for the alpha- Ga2O3 phase for the experiments performed in quasihydrostatic compression conditions. When for the same experiment K-0' is held at 4, then the bulk modulus values are 184(3) and 252 ( 14 ) GPa for beta-Ga2O3 and the alpha-Ga2O3, respectively. We compare the results of this work with our previous studies on the high- pressure behavior of nanocrystalline gallium oxide.