Point defects and their effect on dielectric permittivity in strontium titanate ceramics

The origin of dielectric properties of strontium titanate ceramics is investigated using DFT calculations in periodic system. It was determined that the main factors contributing to the increase in dielectric permittivity are: tetragonal distortion of the normally cubic lattice, and charge imbalance induced displacement of titanium center from its central position. Oxygen vacancies were determined to create significantly larger effects than other types of vacancies, like Ti and SrO. The extent of tetragonal distortion was found to be determined by oxygen vacancy distribution, rather than total concentration: relatively symmetrical distribution of oxygen vacancies resulted in smaller tetragonal distortion of the lattice, and, consequently, smaller increase in dielectric permittivity. Charge imbalance naturally destabilizes the cubic lattice, forcing the Ti-atom out of its central position, resulting in tetragonal lattice with increased dielectric permittivity. The process stabilizes the strontium titanate lattice, while increasing the c/a ratio. Therefore, the dielectric permittivity of strontium titanate can be increased by changes to the system that increase tetragonal distortion of the lattice and/or introduce additional negative charge.

Ab Initio Study of the Effect of Mono-Vacancies on the Metastability of Ga-Stabilized δ-Pu

Most experimental studies on metallic Pu are on the room temperature monoclinic α-phase or the fcc Ga stabilized δ-phase. Stabilized δ-phase Pu-Ga alloys are metastable and exhibit a martensitic phase transformation to α’-phase at low temperatures, or applied shear, with concentrations lower than three atomic percent Ga. By using first principles, we explore the metastability of δ-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.