The Influence of liquid-liquid phase transition in molten argon on the controversial melting results of Mo, Ta and W

Abstract

The long standing controversy between the melting of the bcc Mo,Ta,W and vanadium (V) metals, measured by dynamic shock experiments compared to the melting results obtained by diamond anvil cells has remained an enigma during the last two decades. The isobaric-like behavior of Mo,Ta,W and V under pressures is explained by the thermal expansion of these metals, which compress the liquid or solid argon transmitting medium. Argon, confined in the diamond anvil chamber, compressed by the thermal expansion of the Mo,Ta and W samples, shows evidence of a liquid-liquid phase transition observed by density (viscosity) change upon increasing pressure and the temperature. Two viscous regions are detected: below ~40 GPa where dTm/dP > 0, and above ~40 GPa where dTm/dP → 0. It is claimed that the discrepancy between the dynamic and the static measurements comes from the extremely compressibility of the liquid Ar pressure transmitting medium at pressures above 40(5) GPa and temperatures above 3000 K. The compressed liquid argon at these pressures and temperatures conditions seems to be the most reported compressible liquid in nature. The long standing enigmatic controversy mentioned above is thus solved.