Editorial: The Role of Non-stoichiometry in the Functional Properties of Oxide Materials

On the formation and diffusion of oxygen vacancies in non-stoichiometric mixed Co3-xMnxO4spinel structures from first-principles calculations

Using first-principles simulations, we focus on the study of Co₃O₄-Mn₃O₄mixed oxides, which have recently shown alluring features as thermochemical heat storage materials. We provide fundamental atomistic-level insight into the thermodynamics and kinetics of a series of non-stoichiometric Co_3-xMn_xO_4-y(0 ⩽x⩽ 3 andy= 0, 0.125, 0.250) bulk systems, by examining in detail the formation and diffusion processes of oxygen vacancies as a function of Mn content. We find a preference for the formation of vacancies atx= 1.5. And we predict a significant drop of diffusion barriers forx⩾ 1.5, when Mn atoms start to populate the spinel octahedral sites as Mn³⁺. Our results pave the way for better understanding the underlying mechanisms that govern oxygen vacancy dynamics in Co_3-xMn_xO₄in general, and, in particular, the reversible reduction and re-oxidation reactions of these promising mixed oxides for thermal energy storage. Nevertheless, some discrepancies are found between our calculations on bulk models and recent experimental insights from the literature, which suggests that surface and finite size effects might play an important role in controlling the observed macroscopic behavior of these materials during reversible reduction and re-oxidation cycles.

Ab initio description of oxygen vacancies in epitaxially strained [Formula: see text] at finite temperatures

Epitaxially grown [Formula: see text] (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 ([Formula: see text]), 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 [Formula: see text] 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 [Formula: see text] over wide epitaxial strain conditions ([Formula: see text]%). 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 [Formula: see text]. Furthermore, it is shown that thermal lattice excitations noticeably affect the energy barriers for oxygen ion diffusion, which strongly depend on [Formula: see text] 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.