Novel insights into vacancy effects on high-pressure melting of W from anharmonic statistical moment method

Abstract

W is a ubiquitous refractory metal with numerous potential applications, but its high-pressure melting behaviors remain controversial. Some physicists suggest that the problem may be addressed by including the contribution of vacancies. However, their ideas stay unverified due to the lack of information about crystallographic defects in harsh environments. Herein, the statistical moment method is enhanced to illuminate how vacancies form and impact the melting properties of W. Our anharmonic calculations indicate that the well-known Arrhenius law fails to model the proliferation of vacancies during isobaric heating. Besides, the Gibbs energy of vacancy formation will be severely underestimated if the contribution of external work is neglected during isothermal squeezing. After scrutinizing both temperature and pressure effects, we conclude that the equilibrium vacancy concentration of W drops dramatically along the solid-liquid boundary. Therefore, the Gorecki criterion cannot be applied to describe the melting phenomenon of W. We also demonstrate that the vacancy-mediated melting model of Errandonea is inappropriate for explaining the emergence of abnormally low melting signals in diamond-anvil-cell measurements. Instead, the carbon contamination of W samples should be thoroughly considered to resolve the discrepancy between experimental and computational results. Finally, we revisit the vacancy correction technique of Hung et al. and find that it only depresses the melting temperature by a few percent. The listed pieces of evidence show that the influences of vacancies can be safely ignored when investigating the melting process of W under extreme conditions.