Persistence of Liquid Bismuth Structure Along the Melting Curve Amid Transitions in the Solid Phases

Elemental metal liquids often retain structural characteristics of their corresponding solid phases, exhibiting locally favored arrangements similar to their crystalline phase. Bismuth (Bi) has been extensively studied at ambient pressure, where its melting behavior is well understood. Under high pressure, Bi exhibits five distinct solid phases, four of which are stable along an anomalous melting line, where the melting temperature initially decreases before increasing. However, the structure of liquid Bi at high pressures remains largely unexplored, leading to uncertainty in its phase behavior.

In this study, we investigate the structure of liquid Bi in the 1-4 GPa pressure range, just above the melting line, across multiple solid phase transitions. Energy-dispersive X-ray diffraction (EDXRD) measurements were conducted at the PSICHÉ beamline of the SOLEIL synchrotron, and the collected data were analyzed to obtain the structure factor, S(Q), and the pair distribution function, g(r), at each pressure point. Our results indicate that the short-range order (SRO) in liquid Bi remains largely unchanged despite the transitions between different solid phases. Additionally, the coordination number does not exhibit significant variations that would suggest a corresponding shift in SRO. Further analysis within the Quasi-Crystalline Model (QCM) framework reveals that the SRO in liquid Bi is a distorted Bi-I like-structure, with only minor adjustments. Specifically, the α angle shifts from 49° at ambient pressure to 45° at pressures up to 4 GPa. These findings indicate that, although certain elemental metals may display a direct correlation between their solid-state structures and the structural characteristics of their near-melting liquid phase, this relationship is not universal and does not apply to bismuth.