Freestanding nanocrystalline ␤-Ga 2 O 3 particles with an average grain size of 14 nm prepared by chemical method was investigated by angle-dispersive synchrotron x-ray diffraction in diamond-anvil cell up to 64.9 GPa at ambient temperature. The evolution of x-ray diffraction patterns indicated that nanocrystalline monoclinic ␤-Ga 2 O 3 underwent a phase transition to rhombohedral ␣-Ga 2 O 3 . It was found that ␤to ␣-Ga 2 O 3 transition began at about 13.6-16.4 GPa, and extended up to 39.2 GPa.

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... At the highest pressure used, only ␣-Ga 2 O 3 was present, which remained after pressure release. A Birch-Murnaghan fit to the P-V data yielded a zero-pressure bulk modulus at fixed B 0 Ј=4: B 0 = 228͑9͒ GPa and B 0 = 333͑19͒ GPa for ␤-Ga 2 O 3 and ␣-Ga 2 O 3 phases, respectively. We compared our results with bulk ␤-Ga 2 O 3 , and concluded that the phase-transition pressure and bulk modulus of nanocrystalline ␤-Ga 2 O 3 are higher than those of bulk counterpart.