Microscopic Link between Electron Localization and Chemical Expansion in AMnO₃ and ATiO₃ Perovskites (A = Ca, Sr, Ba)

Abstract

The microscopic origin of chemical expansion in perovskite oxides, due to formation of oxygen vacancies accompanied by formal reduction of a 3d transition metal, is studied by first-principles calculations. We compare the II–IV manganite and titanate series, having Ca, Sr, or Ba on the A site. In particular, the effect of electron localization is elucidated by systematically varying the Hubbard U , and we find that the localization behavior is significantly different in the manganites and titanates. The chemical expansion is explicitly calculated for all compounds, and we demonstrate that increasing on-site repulsion (Hubbard U ) on the B site in the lattice yields increased chemical expansion in the manganites and reduced chemical expansion in the titanates. The opposite behavior of the manganites and titanates arises from different electrostatic screenings of oxygen vacancies. We show that this can be attributed to differences in electronic energy levels, specifically that Mn–O bonds are more covalent than Ti–O bonds. Fundamental understanding of electronic and crystal chemical origins of the important phenomenon of chemical expansion is required for rational design of oxide materials for energy technology, sensors, and actuators. We hope our analysis will inspire further fundamental studies of other oxides for solid state ionics applications.

Chemical lattice strain in nonstoichiometric oxides: an overview

“…167 However, the inuence of charge localization on chemical expansion of perovskites seems to be rather more complex. As was demonstrated recently by Marthinsen et al, 168 depending on the electrostatic screening of oxygen vacancies, determined by the nature of B-site cations, chemical expansion in some perovskites may even be reduced by an increase in the Hubbard U term.…”

“…chemical deformation due to targeted change of the oxygen content in the strained lm can serve as a strain mediation mechanism. 168,[282][283][284] However, in case of thin lms as compared to bulk samples the situation is complicated by anisotropic lattice distortions around oxygen vacancies inherent to many ABO 3 perovskites. 173,174 Elastic interactions of vacancy dipoles with an epitaxial mist strain lead to preferential location of oxygen vacancies in a denite type of atomic plane and, as a result, may cause vacancy ordering, formation of new phases, or phase coexistence/separation.…”

Effects of state filling and localization on chemical expansion in praseodymium-oxide perovskites

“…The U value can also be varied to study chemical expansion over a range of electron localizations, as carried out in previous computational studies. 18,21 Prior to our work, no systematic Hubbard U study of the Pr-4f states for our Pr-based perovskites had been performed. Therefore, we studied the chemical expansion of our Pr-based perovskites over varying U in order to identify physically reasonable U values for our materials.…”

Tuning the Thermoelectric Performance of CaMnO3-Based Ceramics by Controlled Exsolution and Microstructuring

“…The projector-augmented wave (PAW) method was employed to describe the interactions between the core and the valence electrons. Self-interaction errors for Mn d-electrons were addressed by applying a Hubbard U of 3 eV, following previous work on CaMnO 3−δ . , Antiferromagnetic order on the Mn sublattices was imposed for both CaMnO 3−δ and CaMn 2 O 4 . − …”

“…Self-interaction errors for Mn d-electrons were addressed by applying a Hubbard U of 3 eV, 29 following previous work on CaMnO 3−δ . 32,33 Antiferromagnetic order on the Mn sublattices was imposed for both CaMnO 3−δ and CaMn 2 O 4 . 34−37 Structural optimizations of bulk CaMnO 3−δ and CaMn 2 O 4 were performed using plane-wave energy cut-offs of 500 eV and a Γcentered k-point mesh of 4 × 4 × 3 and 4 × 3 × 1, respectively, allowing atomic positions and lattice vectors to relax until the forces on all the atoms were less than 0.01 eV Å −1 .…”