Energy dispersive x-ray diffraction and reverse Monte Carlo structural study of liquid gallium under pressure

The structure of liquid gallium has been studied along the melting curve from 0.64 to 5.6 GPa by the energy dispersive x-ray diffraction technique, followed by modeling of the experimental data by the reverse Monte Carlo (RMC) method. The RMC models were constrained by the experimentally obtained equation of states of liquid Ga and in good accordance with experimental data. Analysis of the structure factor $S(Q)$ and the radial distribution function $g(r)$ shows that the anisotropic local structure of liquid Ga deviates from that of a simple hard-sphere-like liquid metal structure. Whereas the third and fourth coordination shell positions and position of the first maximum of $S(Q)$ demonstrate pressure dependencies close to a uniform compression scaled by the ${(V/{V}_{0})}^{1/3}$ volume relation, the positions of the first (especially) and second coordination spheres have more flat pressure dependencies. At the same time, the first and second coordination numbers increase: The first coordination number starts from 10 to 10.5 and increases by $\ensuremath{\sim}$5$%$ in the studied pressure interval. This indicates that liquid gallium contraction is nonuniform, and the local structure changes with increasing pressure. Analysis of the radial distribution function $g(r)$ by a distorted-crystalline model shows that at lower pressures liquid consists of two species similar to the solid Ga I and Ga II structures. The fraction of the Ga I-like part is about $0.2\ifmmode\pm\else\textpm\fi{}0.05$ at 0.64 GPa, and it gradually decreases under pressure to zero at approximately $7.5\ifmmode\pm\else\textpm\fi{}0.5$ GPa.