Structure determination and stability for Pa-Si, Np-Si and U-X-Si (X = Mo, Th, Np) phases from first-principles

Abstract

Density functional theory (DFT) calculations are performed for Pa-Si, Np-Si and uranium-based ternary silicide phases. Structure prediction calculations are used to search for competing phases in these systems. Results using the generalized gradient approximation (GGA), on-site Coulomb correction (GGA + U) and van der Waals interactions are presented. All Pa-Si compounds reported here are structurally analogous to those found in other actinide silicide systems. The electronic structure of Pa3Si2 shows the f-orbital electrons are largely unoccupied, which is in contrast to calculations for Np3Si2. For the Np-Si system, predicted stable structures using GGA differ from the experimentally observed structures, which, however, are energetically preferred in results using the GGA + U method. Structure searches for U2MoSi, U2ThSi2 and UNpSi reveal dynamically stable ternary compounds. The phonon dispersion curves, elastic constants and electronic density of states for the various phases are compared to those from previous DFT calculations for U-Si phases.