Stability of c-BAs under extreme conditions: An ab initio molecular dynamics study

Recent research has highlighted a remarkably large thermal conductivity and high ambipolar mobility in cubic boron arsenide (c-BAs), positioning it as a promising heat control material substituting diamond for future semiconductor technologies with intensified power and elevated temperatures. However, many thermodynamic properties of c-BAs under high temperature and pressure remain unexplored, one of which is its poorly understood melting behavior. While a few early experiments have reported thermal instability of c-BAs above 1200 K ($920{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$), a much higher melting temperature ($>2000\phantom{\rule{4pt}{0ex}}\mathrm{K}$) was predicted by recent theoretical estimates via an empirical formula relying on the elastic constants of c-BAs calculated at 0 K. In this work, we investigate the phonon stability, Born mechanical stability, deformation stability under large strains of c-BAs at high temperature, together with its pressure-dependent melting behavior, using ab initio molecular dynamics (AIMD) simulations. The results reveal that the equilibrium structure of c-BAs is dynamically and mechanically stable up to 1900 K. Although the mechanical strength is found to reduce approximately by half as the temperature increases from 300 to 1900 K, c-BAs remains stable at 1900 K when the (tensile, compression, and shear) deformation strains are within 0.1. Moreover, the ultralarge elasticity observed in nanosized c-BAs under large deformation strains at room temperature can mostly preserve up to a high working temperature around $1000{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. The melting temperature of c-BAs determined by our AIMD solid-liquid coexistence tests is $1900\ifmmode\pm\else\textpm\fi{}100\phantom{\rule{4pt}{0ex}}\mathrm{K}$, close to those predicted by the empirical formula. Under applied pressures, the melting temperature of c-BAs reveals a marked reentrant behavior, which initially increases with pressure but then decreases when the pressure exceeds 12.5 GPa, similar to the observed unique pressure dependence of its thermal conductivity. However, detailed analyses unveil that the reentrant melting behavior of c-BAs comes from the special liquid state of BAs where boron atoms form dense clusters so that the specific volume of liquid BAs quickly becomes smaller than that of solid BAs under a moderate pressure of 12.5 GPa, while a pressure as high as 450 GPa is needed to observe the reentrant melting behavior of diamond. These findings provide more deepened understandings in the mechanical stability and melting dynamics of c-BAs under extreme conditions, which are essential for its potential applications in advanced semiconductor chips and as efficient thermal conductive substrates.