Ab Initio Study Of The Effect Of Mono-Vacancies On The Metastability Of Ga-Stabilized Delta-Pu

Most experimental studies on metallic Pu are on the room temperature monoclinic alpha-phase or the fcc Ga stabilized delta-phase. Stabilized delta-phase Pu-Ga alloys are metastable and exhibit a martensitic phase transformation to alpha'-phase at low temperatures, or applied shear, with concentrations lower than three atomic percent Ga. By using first principles, we explore the metastability of delta-phase by investigating the structural and electronic behavior induced by Ga alloying and by a mono-vacancy point defect. We find that a site substitutional Ga induces a tetragonal distortion in the lattice affected by hybridization of Ga 4p and Pu 6d states. With the addition of a mono-vacancy, a monoclinic or tetragonal distortion forms locally (dependent on its distance from Ga), and decoupling of the Pu 5f and 6d states and broadening of the 6d states occurs. This response enables hybridization of Pu 6d with the Ga 4p states affecting the mono-vacancy formation energy. Thus, stabilization of the fcc lattice correlates with hybridization of Pu 6d states with Ga 4p states, and this becomes more evident in the presence of a mono-vacancy.