Structure of Liquid Mercury at High-Pressure

The atomic-scale structure and melting curve of liquid mercury was measured using in situ synchrotron x-ray diffraction (SXRD) at pressure and temperature (p-T ) conditions up to 9.44(2) GPa and 651(1) K. Ab initio molecular dynamics (AIMD) simulations were employed to obtain a detailed atomistic model of the liquid structure. The results reveal a pronounced flattening, and potential maximum, in the measured melting curve between 6 and 9 GPa. The structure factors S_HgHg (Q) and pair distribution functions g_HgHg (r) calculated from the AIMD simulations are in good overall agreement with the SXRD measurements under comparable reduced densities and temperatures, indicating that the atomistic structure of liquid Hg is well captured by AIMD. With increasing pressure, the principal peak in S_HgHg (Q) shifts to higher Q, with the subsidiary peak at Q = 2k_F experiencing a concomitant shift consistent with the increased electron density. Considering the Evans t-matrix formulation of the Ziman theory of liquid metals, the structural S(2k_F) term is expected to have only a weak influence on the electrical resistivity under compression. In contrast, the pressure-induced broadening and shift of the d-projected density of states towards the Fermi level is consistent with enhanced near-resonant d-electron scattering, and a corresponding increase in resistivity, analogous to the behavior of first-row transition metals. Analysis of the measured g_HgHg (r) functions, and AIMD trajectories in real-space, indicates that the liquid structure experiences a progressive development towards simple hard-sphere-like behaviour at increasing p-T along the melting curve. However, topological cluster classification analysis shows that while the structural fingerprint of liquid Hg strongly resembles an effective hard-sphere system, even at the highest pressures investigated it contains more many-body motifs than expected for this simple model.