Influence of strain on the structural instabilities and functional properties of complex oxides

Complex oxides display a tremendous array of functional properties, ranging from ferroelectricity to giant magnetoresistance to superconductivity. Epitaxial strain in thin films of these materials provides a tool to further manipulate the available functionalities and presents an attractive avenue for material design. Arising from mismatch between films and substrate lattice parameters, strain can have profound effects on material properties by altering the energy balance between competing structural instabilities. Here we use density functional calculations to address the influence of strain on ABO 3 perovskite oxides, examining the impact on structural instabilities and focusing on regions in which strain induces phase transition phenomena. The density functional approach gives us access to the atomic-level details of a material's response to strain, enabling us to disentangle competing instabilities and differentiate between the ionic, electronic and lattice responses. In particular, we seek to illuminate the coupling of strain to distortion modes involving rigid rotations of the BO6 octahedral units. These octahedral rotations are known to drive or prohibit a number of strain-induced phenomena in functional oxides, but the details of their coupling to strain were not previously well understood.;We develop our topic through investigations on four perovskite systems. We first study a layered superlattice of La(Al,Fe,Cr)O3, to distinguish the effect of misfit strain from the symmetry constraint imposed by heterostructuring. We then focus on the influence of strain in a series of single-phase perovskites of increasing complexity. In the simple dielectric LaAlO3, we isolate the strain-rotation coupling, develop a model for the dependence of rotations on bi-axial and uni-axial strain, and characterize a previously unidentified strain-induced phase transition. Next, we address strain in a polar material, multiferroic BiFeO3, to investigate recent experimental reports of a high-strain phase, and we identify an unusual isosymmetric transition corresponding to a destabilized rotational mode and a massive increase in the axial ratio. Finally, we include the effects of strong orbital ordering in a study of the strain response of BiMnO3. Our results contribute to a solid theoretical understanding of the influence of strain on functional oxides to facilitate continued progress in the field of strain engineering.