The Conundrum of Relaxation Volumes in First-Principles Calculations of Charged Defects in UO2

Anuj Goyal,Simon R Phillpot,Richard G Hennig,Christopher R Stanek,Samuel T Murphy,Blas P Uberuaga,Aleksandr Chernatynskiy,Kiran Mathew,David A Andersson

doi:10.3390/app9245276

Abstract

The defect relaxation volumes obtained from density-functional theory (DFT) calculations of charged vacancies and interstitials are much larger than their neutral counterparts, seemingly unphysically large. We focus on UO2 as our primary material of interest, but also consider Si and GaAs to reveal the generality of our results. In this work, we investigate the possible reasons for this and revisit the methods that address the calculation of charged defects in periodic DFT. We probe the dependence of the proposed energy corrections to charged defect formation energies on relaxation volumes and find that corrections such as potential alignment remain ambiguous with regards to its contribution to the charged defect relaxation volume. We also investigate the volume for the net neutral defect reactions comprising individual charged defects, and find that the aggregate formation volumes have reasonable magnitudes. This work highlights the issue that, as is well-known for defect formation energies, the defect formation volumes depend on the choice of reservoir. We show that considering the change in volume of the electron reservoir in the formation reaction of the charged defects, analogous to how volumes of atoms are accounted for in defect formation volumes, can renormalize the formation volumes of charged defects such that they are comparable to neutral defects. This approach enables the description of the elastic properties of isolated charged defects within an overall neutral material.

Highlights

Charged defects in non-metals, such as semiconductors and insulators, play a central role in determining electronic and electrical behavior of materials for semiconductor and battery applications [1].Point defects are very relevant to actinide materials such as UO2 under extreme conditions of temperature, pressure, and irradiation, as they play a major role in accommodating stoichiometry variations [2]
To ensure there are no issues associated with the change in plane wave basis and k-point density due to changing cell size, we calculate the defect relaxation volume of point defects for higher energy cut-off and denser k-point mesh; we find that the relaxation volumes of neutral and charged defects are converged to 0.3–1.8 Å3 depending on the type of defect
We find that the changes in the formation energy differ significantly between the charged and neutral states for a given defect

Summary

Introduction

Charged defects in non-metals, such as semiconductors and insulators, play a central role in determining electronic and electrical behavior of materials for semiconductor and battery applications [1].Point defects are very relevant to actinide materials such as UO2 under extreme conditions of temperature, pressure, and irradiation, as they play a major role in accommodating stoichiometry variations [2]. Sci. 2019, 9, 5276 volume which can, at least in principle, be measured experimentally [3,4] and compared to theoretical calculations [4,5,6,7,8,9]. These lattice distortions are important in determining the interaction of defects with external strains, dislocations, and grain boundaries [10,11], and play a central role in the evolution of microstructures in materials

Results

Discussion

Conclusion