The temperature dependence of bismuth structures under high pressure

It is unclear whether there is a liquid-liquid phase transition or not in the bismuth melt at high temperature and high pressure, if so, it is necessary to confirm the boundary of the liquid-liquid phase transition and clarify whether it is a first-order phase transition. Here based on X-ray absorption spectra and simulations, the temperature dependence of bismuth structures has been investigated under different pressures. According to the similarity of characteristic peaks of X-ray absorption near edge structure (XANES) spectra, we have estimated that the possible temperature ranges of liquid-liquid phase transition are 779 K ~ 799 K at 2.74 GPa and 859 K ~ 879 K at 2.78 GPa, 809 K ~ 819 K at 3.38 GPa and 829 K ~ 839 K at 3.39 GPa and 729 K ~ 739 K at 4.78 GPa, respectively. Using ab initio molecular dynamics (AIMD) simulations, we have obtained the stable structures of the bismuth melt at different temperatures and pressures and calculated their electronic structures. Meanwhile, two stable phases (phase III-like and phase IV-like) of bismuth melts are obtained from different initial phases of bismuth solids (phase III and phase IV) under the same condition (3.20 GPa and 800 K). Assuming that the bismuth melt undergoes a phase transition from IV-like to III-like between 809 K and 819 K at 3.38 GPa, the calculated electronic structures are consistent with XANES spectra, which provides a possible explanation for the first-order liquid-liquid phase transition.