The conundrum of relaxation volumes in first-principles calculations of charge defects

TL;DR

This paper investigates why charged defect relaxation volumes calculated via DFT are unphysically large, examining correction methods and proposing a volume renormalization approach to better understand charged defect elastic properties.

Contribution

It introduces a volume renormalization method considering electron reservoir changes, improving the physical interpretation of charged defect relaxation volumes in DFT calculations.

Findings

Correction methods have negligible effect on relaxation volumes.

Aggregate formation volumes of neutral reactions are reasonable.

Volume renormalization aligns charged defect volumes with neutral defect expectations.

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. In this work, we investigate the possible reasons for this and revisit the methods that address the calculation of charged defect structures 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 the image charge correction and the alignment correction, which can lead to sizable changes in defect formation energies, have an almost negligible effect on the charged defect relaxation volume. We also investigate the volume for the net neutral defect reactions comprised of individual charged defects, and find that the aggregate formation volumes have reasonable magnitudes. This work highlights an important issue that, as 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 the overall neutral material, beyond the context of the overall defect reactions that produce the charged defect.