We use first-principles calculations based on density functional theory to investigate the interplay between oxygen vacancies, A-site cation size/tolerance factor, epitaxial strain, ferroelectricity, and magnetism in the perovskite manganite series, AMnO 3 (A = Ca 2+ , Sr). We find that, as expected, increasing the volume through either chemical pressure or tensile strain generally lowers the formation energy of neutral oxygen vacancies consistent with their established tendency to expand the lattice. Increased volume also favors polar distortions, both because competing rotations of the oxygen octahedra are suppressed and because Coulomb repulsion associated with cation off-centering is reduced. Interestingly, the presence of ferroelectric polarization favors ferromagnetic (FM) over antiferromagnetic (AFM) ordering due to suppressed AFM superexchange as the polar distortion bends the Mn-O-Mn bond angles away from the optimal 180°. Intriguingly, we find that polar distortions compete with the formation of oxygen vacancies, which have a higher formation energy in the polar phases; conversely the presence of oxygen vacancies suppresses the onset of polarization. In contrast, oxygen vacancy formation energies are lower for FM than AFM orderings of the same structure type. Our findings suggest a rich and complex phase diagram, in which defect chemistry, polarization, structure, and magnetism can be modified using chemical potential, stress or pressure, and electric or magnetic fields.

The structural and electronic response of LaAlO3 to biaxial strain in the (111) plane is studied by density functional theory (DFT) and compared to strain in the (001) plane and isostatic strain. For (111)-strain, in-plane rotations are stabilized by compressive strain and out-of-plane rotations by tensile strain. This is an opposite splitting of the modes compared to (001)-strain. Furthermore, for compressive (111)strain in-plane rotations are degenerate with respect to rotation axis, giving rise to Goldstone-like modes. We rationalize these changes in octahedral rotations by analyzing the VA/VB polyhedral volume ratios. Finally, we investigate how strain affects the calculated band gap, and find a 28 % difference between the strain planes under 4 % tension. This effect is attributed to different A-site dodecahedral crystal field splitting for (001)-and (111)-strain.

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Strain-phonon coupling, in terms of the shift in phonon frequencies under biaxial strain, is studied by density functional theory calculations for twenty perovskite oxides strained in their (111)-and (001)-planes. While the strain-phonon coupling under (001)-strain follows the established, intuitive trends, the response to (111)-strain is more complex. Here we show that strain-phonon coupling under (111)-strain can be rationalized in terms of the Goldschmidt tolerance factor and the formal cation oxidation states. The established trends for coupling between (111)-strain and in-phase and out-of-phase octahedral rotational modes as well as polar modes provide guidelines for rational design of (111)-oriented perovskite thin films.2

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.