High-pressure x-ray diffraction studies of the nanostructured transparent vitroceramic medium K₂O-SiO₂-Ga₂O₃

Synchrotron-radiation-based, energy-dispersive x-ray-diffraction studies have been performed on a composite containing nanometer-size aggregates embedded in an amorphous matrix, in the pressure range from ambient up to 15 GPa. The optically transparent material containing β-Ga₂O₃ nanocrystals was developed by the controlled crystallization of a silicon oxide-based amorphous precursor. Transmission electron microscopy and conventional x-ray-diffraction techniques allowed estimating the mean size of a single-crystalline phase to be 14.8±1.9 nm, distributed homogeneously in an amorphous medium. The pressure-driven evolution of x-ray-diffraction patterns indicated a progressive densification of the nanocrystalline phase. A structural modification corresponding to a pressure-induced coordination change of the gallium atoms was evidenced by the appearance of new diffraction peaks. The overall changes of x-ray-diffraction patterns indicated a β-Ga₂O₃ to α-Ga₂O₃ phase transformation. The low- to high-density phase transition was initiated at around 6 GPa and not completed in the pressure range investigated. A Birch-Murnaghan fit of the unit-cell volume change as a function of pressure yielded a zero-pressure bulk modulus, K₀, for the nanocrystalline phase of 191±4.9 GPa and its pressure derivative, K₀′=8.3±0.9.