Self-consistent DFT+U+V study of oxygen vacancies in SrTiO3

Abstract

Contradictory theoretical results for oxygen vacancies in SrTiO 3 (STO) were often related to the peculiar properties of STO, which is a d 0 transition metal oxide with mixed ionic-covalent bonding. Here, we apply, for the first time, density functional theory (DFT) within the extended Hubbard DFT + U + V approach, including onsite as well as intersite electronic interactions, to study oxygen-deficient STO with Hubbard U and V parameters computed self-consistently via density-functional perturbation theory. Our results demonstrate that the extended Hubbard functional is a promising approach to study defects in materials with electronic properties similar to STO. Indeed, DFT + U + V provides a better description of stoichiometric STO compared to standard DFT or DFT + U , the band gap and crystal field splitting being in good agreement with experiments. In turn, also the description of the electronic properties of oxygen vacancies in STO is improved, with formation energies in excellent agreement with experiments as well as results obtained with the most frequently used hybrid functionals, however, at a fraction of the computational cost. While our results do not fully resolve the contradictory findings reported in literature, our systematic approach leads to a deeper understanding of their origin, which stems from different cell sizes, STO phases, the exchange-correlation functional, and the treatment of structural relaxations and spin-polarization.

Electronic structure of pristine and Ni-substituted LaFeO3 from near edge x-ray absorption fine structure experiments and first-principles simulations

“…The present implementation of DFT + U + V allows for calculations of XANES spectra in a broad energy range thanks to the use of the Lanczos recursive method, which removes the need of computing empty states which otherwise would be needed when standard DFT-based techniques are used. As will be shown, the use of extended Hubbard functionals allows us to refine some low-energy features in the XANES spectra at a computational cost comparable to standard DFT [58,71], finding that the prepeak in the oxygen K-edge spectrum of LFNO is of mixed Ni(3d )-O(2p) character, in agreement with experimental findings of Refs. [21,32].…”

“…[70] to the case of extended Hubbard functionals, i.e., the DFT + U + V approach. The inclusion of both onsite and intersite Hubbard interactions allows us to obtain an accuracy for the low-energy features of XANES spectra comparable to that of hybrid functionals but at substantially lower computational cost [58,71].…”

“…In many of these systems the onsite Hubbard U correction alone fails to accurately predict the electronic occupations and energies when TM elements with different oxidation states are present simultaneously, and where intersite V is truly fundamental to restore the correct balance between localization of states and their hybridization. Moreover, whenever perovskite materials contain oxygen vacancies or defects, once again intersite Hubbard interactions are absolutely necessary to describe accurately the formation energies and energetics in general [58], and which thus are expected to be of crucial importance for calculations of XANES spectra. Therefore, further applications of the novel XANES method based on DFT + U + V are envisaged to clarify further the impact of intersite interactions on XANES spectra of complex TM compounds.…”

“…While empirical evaluations of U are quite common, various methods exist to compute it from first principles [53][54][55][56], which render this method fully ab initio. In particular, an extension to include the effects of the intersite V electronic interactions (DFT + U + V ) has been introduced quite recently [45], with the intersite term being crucial for the energetics of complex TM oxides with covalent interactions (i.e., with significant hybridization between, e.g., the d and p states of neighboring sites) [57,58].…”

Role of nonlocality in exchange correlation for magnetic two-dimensional van der Waals materials

“…Thus, there is no way that the DFT + U can be used to mimic QSGW for these systems. It would be interesting to see whether extended Hubbard models, such as DFT + U + V [53], can satisfactorily approximate such off-site correlations, especially with parameters determined systematically and automatically.…”