Gallium melts under high-pressure and temperature conditions: Synchrotron x-ray tomography and diffraction studies

Abstract:

Gallium has a rare water-ice type P-V-T phase diagram in which the density of Ga melt exceeds by about 3 % that of the stable solid Ga phase I at ambient pressure. Liquid gallium, which is easily supercooled, can remain in a metastable liquid state for several months at ambient pressure. A rich polymorphism and metastable modifications of Ga have been discovered in P-T domain. There are a number of studies of liquid gallium under high pressure conditions, but some fundamental properties, such as the equation of state (EoS) of liquid Ga under extreme conditions remain unclear. Very recently, the advanced pair distribution function (PDF) method in which synchrotron high-energy x-ray total scattering data, combined with reverse Monte Carlo simulation, was used to study the microstructure and EoS of liquid gallium under high pressure conditions. However, the application of PDF method for amorphous or liquid samples under pressure conditions normally required a priori knowledge of their EoS. The density estimation from the reverse Monte Carlo simulation with the best mathematical fit to the measured structure factor data could cause big errors if it is calculated without knowing the EoS. In the paper, the volume change of liquid and solid gallium have been studied as a function of pressure and temperature up to 3.63 GPa using synchrotron x-ray microtomography combined with energy dispersive x-ray diffraction (EDXRD) techniques. Two sets of directly measured P-V data at 300 K and 330 K were obtained from 3D tomography reconstruction data. The existence of a liquid-liquid phase transition region is proposed based on the abnormal compressibility of Ga melt at about 2.44 GPa and 330 K conditions.