Ab initio study and thermodynamic modeling of the Pd-Si-C system

Understanding phase stability in the Pd-Si-C ternary system is important for predicting the high-temperature behavior of tristructural isotropic (TRISO) nuclear fuels for high-temperature gas-cooled reactors. Here, we employ ab initio evolutionary search to predict the structure and energetics of ordered compounds in Pd-Si-C and its binary sub-systems, Pd-Si, Si-C and Pd-C. In addition to confirming the experimentally-known crystal structures for Pd2Si, PdSi, and SiC, our evolutionary search uncovers a previously unreported orthorhombic Pd5Si structure with Cmcm symmetry that is thermodynamically stable at low temperatures. The vibrational formation entropies of Pd3Si, Pd2Si, and PdSi are calculated using the direct force-constant method within the harmonic approximation. Furthermore, ab initio molecular dynamics simulations have been performed to obtain the atomic structures and energetics of liquid alloys. With the incorporation of the present theoretical results, an ab initio informed thermodynamic description of the Pd-Si-C ternary system has been obtained in this study.