Liquid-liquid transition in phosphorus: Temperature evolution of the density jump and search for a liquid-liquid critical point

Two or more liquid states may exist even for single-component substances, and the transition between these states is called a liquid-liquid transition (LLT). The LLT of phosphorus (P) has attracted considerable attention since the direct experimental observation of two fluid phases. However, several important questions remain, such as the variation of the density jump along the transition line, and whether the latter ends at a critical point (LLCP). Moreover, there are at present large discrepancies between different experimental measurements of the melting line, especially in the region of the LLT. The aim of this proposal is to explore the HP-HT phase diagram of P at pressure up to 4 GPa and temperature up to 3000 K, employing X-ray powder diffraction and absorption, to determine precisely the density change across the LLT and melting transitions, and extend the measurement of the LLT to higher T to check for the existence of a LLCP.