Spin density stabilization of local distortions induced by a monovacancy in δ -Pu

Abstract

Density functional theory calculations elucidate crystallographic and electronic structural responses of the fcc delta phase of plutonium (\ensuremath{\delta}-Pu) to point defects. Discussed in this paper are responses that vary from a defect exhibiting a local structural instability common in fcc metals to an unusual defect structure that mimics properties similar to the monoclinic \ensuremath{\alpha} phase. In the prior case, the existence of a self-vacancy when ionic relaxation is allowed induces in the fcc lattice a local tetragonal instability in nearest-neighbor atoms, slight electronic charge increase, and a spin-density decrease. However, in the latter case, after ionic relaxation, the defect is not recognizable as the structure of a vacant site common to many simple fcc metals, but is a complex extended defect involving neighboring atoms to collectively exhibit loss of lattice symmetry through formation of short Pu-Pu bonds and associated narrow bands at the Fermi energy. Partial density of states (PDOS) indicates that these narrow bands form as spectral weighting from the less energetic $6d$ electronic states is shifted to the $5f$ electronic states by means of the spin density, which occupies states at the Fermi energy. The PDOS of a $5f$ system exhibiting narrow bands at the Fermi energy is associated with the formation of an unusual defect structure in the lattice.