Anomalous melting behavior of polycrystalline bismuth quenched at high temperature and high pressure

Under ambient conditions, bismuth (Bi) adopts a rhombohedral A7 structure, yet Bi treated at high temperature and high pressure (HTHP) has been proven to have derivative structural polytypes that coexist with the common A7 structure. This paper studies the melting behavior of HTHP-treated polycrystalline Bi using differential scanning calorimetry (DSC) and in situ transmission electron microscopy (TEM) measurements. The Bi samples quenched at a pressure of 2 GPa and a temperature above 2000 °C show an additional endothermic peak before the common melting peak of A7-Bi in the DSC curves. This is probably due to the high-temperature relaxation of the distorted A7 configuration. The samples maintain their initial morphology characteristics during repeated heating cycles, indicating the incomplete melting of distorted Bi polytypes.