Computational Free Energy Analysis Uncovers Anomalous Phase Transformation in Silicon

Silicon is highly significant both technologically and as a structural material. In this study, we explore the melting mechanism of silicon using molecular dynamics (MD) simulations and Stillinger-Weber (SW) potential, with a focus on free energy analysis. A first-order phase transformation from solid-liquid is proposed to understand the various anomalies of Silicon. In contrast to conventional substances, a sudden drop in density has been observed during the quenching process, while an abrupt jump in density is noticed during the heating process. The liquid-liquid transition point(T _LL ) is identified by a change in the first shell coordination number (CN) from 8 to approximately 5 as the temperature decreases. We estimate the liquid-liquid transition point (T _LL ) to be 1375 K. During the quenching process, the tetrahedral order parameter rises sharply from 0.57 to 0.92 at 1050 K, indicating the formation of the diamond crystalline structure. A three-stage reversible thermodynamic cycle is employed to evaluate the free energy between solid and liquid phases, while the multiple histogram reweighting (MHR) technique is used to determine the equation of state for the liquid and solid phases. The value of the free energy difference between the two phases is approximately −0.3520 ± 0.0135 kJ/mol at the approximate melting temperature. The solid-liquid coexistence temperature of Silicon is approximately 1673 ± 5 K. The melting line of Silicon is obtained using Gibbs-Duhem integration from a single coexistence point. The solid-liquid transition curve shows a negative slope, indicating that as pressure rises, the melting point decreases. The typical gradient of the pressure-temperature coexistence line is approximately 61.7 K/GPa.