Abstract
Epitaxially grown hbox {SrTiO}_{{3}} (STO) thin films are material enablers for a number of critical energy-conversion and information-storage technologies like electrochemical electrode coatings, solid oxide fuel cells and random access memories. Oxygen vacancies ({mathrm{V}_{{mathrm{O}}}}), on the other hand, are key defects to understand and tailor many of the unique functionalities realized in oxide perovskite thin films. Here, we present a comprehensive and technically sound ab initio description of {mathrm{V}_{{mathrm{O}}}} in epitaxially strained (001) STO thin films. The novelty of our first-principles study lies in the incorporation of lattice thermal excitations on the formation energy and diffusion properties of {mathrm{V}_{{mathrm{O}}}} over wide epitaxial strain conditions (-4 le eta le +4%). We found that thermal lattice excitations are necessary to obtain a satisfactory agreement between first-principles calculations and the available experimental data for the formation energy of {mathrm{V}_{{mathrm{O}}}}. Furthermore, it is shown that thermal lattice excitations noticeably affect the energy barriers for oxygen ion diffusion, which strongly depend on eta and are significantly reduced (increased) under tensile (compressive) strain. The present work demonstrates that for a realistic theoretical description of oxygen vacancies in STO thin films is necessary to consider lattice thermal excitations, thus going beyond standard zero-temperature ab initio approaches.
Highlights
Crystalline defects, namely, deviations from the ideal periodic arrangement of atoms in crystals, are ubiquitous in real solids
A “Hubbard-U” scheme[44] was employed for a better treatment of the Ti 3d electronic orbitals with a selected U value of 2.0 eV. (The main conclusions presented in this article do not appreciably depend on this particular choice as demonstrated by numerical tests carried out for U = 4 eV, see Supplementary Fig. 1 and "Formation energy of oxygen vacancies at T = 0" section.) The energy band gap of (001) SrTiO3 thin films was accurately estimated with the range-separated hybrid functional HSE0645 for the equilibrium structures previously determined at the GGA-PBE+U level
We start by discussing the zero-temperature phase diagram of stoichiometric (001) STO thin films calculated with first-principles methods
Summary
Crystalline defects, namely, deviations from the ideal periodic arrangement of atoms in crystals, are ubiquitous in real solids. The ferroelectric14–18, magnetic8,19, optical[20,21] and catalytic[12,21] properties of TMO thin films can be drastically changed by strain engineering due to the existing strong couplings between their structural and electronic degrees of freedom. An illustrative example of the rich interplay between epitaxial strain and oxygen vacancies, which in turn may enormously influence the prevalent orbital and structural order parameters, is provided by the archetypal oxide perovskite SrTiO3 (STO). Bulk STO is broadly used as a substrate in which to grow epitaxial perovskite thin films of high quality and as a key component of oxide heterostructures exhibiting fundamentally intriguing physical behaviour (e.g., LaTiO3/STO bilayers, in which a 2D electron gas appears at the interface[27], and PbTiO3/STO superlattices, in which polar vortices have been observed[28]). Within a certain range of biaxial compressive stress, antiferrodistortive (AFD) oxygen octahedra rotations are observed to coexist with out-of-plane electric p olarization[30], pointing to the presence of unusual cooperative couplings between such typically opposing order p arameters[31]
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