Importance of elastic finite-size effects: Neutral defects in ionic compounds

P. A. Burr and M. W. D. Cooper
Phys. Rev. B 96, 094107 – Published 15 September 2017
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Abstract

Small system sizes are a well-known source of error in density functional theory (DFT) calculations, yet computational constraints frequently dictate the use of small supercells, often as small as 96 atoms in oxides and compound semiconductors. In ionic compounds, electrostatic finite-size effects have been well characterized, but self-interaction of charge-neutral defects is often discounted or assumed to follow an asymptotic behavior and thus easily corrected with linear elastic theory. Here we show that elastic effects are also important in the description of defects in ionic compounds and can lead to qualitatively incorrect conclusions if inadequately small supercells are used; moreover, the spurious self-interaction does not follow the behavior predicted by linear elastic theory. Considering the exemplar cases of metal oxides with fluorite structure, we show that numerous previous studies, employing 96-atom supercells, misidentify the ground-state structure of (charge-neutral) Schottky defects. We show that the error is eliminated by employing larger cells (324, 768, and 1500 atoms), and careful analysis determines that elastic, not electrostatic, effects are responsible. The spurious self-interaction was also observed in nonoxide ionic compounds irrespective of the computational method used, thereby resolving long-standing discrepancies between DFT and force-field methods, previously attributed to the level of theory. The surprising magnitude of the elastic effects is a cautionary tale for defect calculations in ionic materials, particularly when employing computationally expensive methods (e.g., hybrid functionals) or when modeling large defect clusters. We propose two computationally practicable methods to test the magnitude of the elastic self-interaction in any ionic system. In commonly studied oxides, where electrostatic effects would be expected to be dominant, it is the elastic effects that dictate the need for larger supercells: greater than 96 atoms.

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  • Received 20 March 2017
  • Revised 11 May 2017

DOI:https://doi.org/10.1103/PhysRevB.96.094107

©2017 American Physical Society

Physics Subject Headings (PhySH)

Condensed Matter, Materials & Applied PhysicsAtomic, Molecular & OpticalGeneral PhysicsAccelerators & BeamsGravitation, Cosmology & AstrophysicsNonlinear DynamicsPhysics Education ResearchQuantum Information, Science & TechnologyStatistical Physics & ThermodynamicsPlasma PhysicsNuclear PhysicsInterdisciplinary PhysicsFluid DynamicsPhysics of Living SystemsNetworksParticles & FieldsPolymers & Soft Matter

Authors & Affiliations

P. A. Burr*

  • School of Electrical Engineering and Telecommunications, University of New South Wales, Kensington, 2052 New South Wales, Australia

M. W. D. Cooper

  • Materials Science and Technology Division, Los Alamos National Laboratory, P.O. Box 1663, Los Alamos, New Mexico 87545, USA

  • *p.burr@unsw.edu.au

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Issue

Vol. 96, Iss. 9 — 1 September 2017

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