Revisiting point defect thermodynamics in group IVB and VB transition metal carbides

We present a comprehensive re-examination of point defect thermodynamics in group IVB and VB transition metal carbides (TMCs) with the rocksalt structure using a combination of density functional theory (DFT) calculations and a statistical mechanical Wagner-Schottky model within the canonical ensemble. The most stable configurations of point defects were discovered using basin-hopping global optimization, driven by either a machine learning interatomic potential (MLIP) or DFT. A key finding is the identification of previously unreported dicarbon antisites—a C–C dimer occupying a metal site—as the structural (constitutional) defects on the carbon-rich side of stoichiometry in all group IVB and VB TMCs except TaC. Furthermore, dicarbon antisite-containing thermal defect complexes, such as quadruple and interbranch defects, can dominate in TMCs under specific stoichiometric and temperature conditions. In conclusion, by incorporating dicarbon antisites into the defect landscape, this work provides a revised understanding of the thermodynamics of point defects in TMCs.. Authors: Jiang, Chao [Idaho National Laboratory (INL), Idaho Falls, ID (United States)] (ORCID:0000000306106327); Aagesen, Larry K. [Idaho National Laboratory (INL), Idaho Falls, ID (United States)] (ORCID:000000034936676X); Gan, Jian [Idaho National Laboratory (INL), Idaho Falls, ID (United States)] (ORCID:0000000291214164). DOE Contract: AC07-05ID14517. Subjects: Carbides; Density functional theory (DFT); Machine learning; Point defects