Energetics and kinetics of metal impurities in the low-temperature ordered phase of V2C from first-principles calculations

Abstract

The site preference of Fe, Ni, Ce, Nd and U impurities and their migration behaviors in the low-temperature ordered orthorhombic phase of V2C are calculated using density functional theory. It is found that all impurities prefer the substitutional vanadium site over the octahedral interstitial site. The energy required to incorporate an impurity into the lattice increases with increasing impurity size. Binding between an impurity at the substitutional vanadium site and a neighboring vanadium vacancy is shown to be much stronger for large impurities, U, Ce and Nd, than for small impurities, Fe and Ni. The strong binding can be attributed to the openness of the V2C structure, which allows large impurity atoms to relax by shifting towards neighboring empty octahedral sites. The proximity of each impurity-vacancy pair to nearby carbon interstitial atoms leads to a high degree of anisotropy in binding strength with binding energies differing by > 2 eV for the same impurity type. Small impurities, Fe and Ni, have negative impurity volumes at the substitutional vanadium site, so binding with neighboring vanadium vacancies is much weaker. Migration barriers for direct exchange of an impurity with a vanadium vacancy are calculated for Fe and Ni. On average migration barriers for Ni are lower than those for Fe.