Interface-induced enhancement of piezoelectricity in the (SrTiO3)m/(BaTiO3)M-m superlattice for energy harvesting applications

The electronic properties of SrTiO3-δ with oxygen vacancies or substitutions

The electronic properties, including bandgap and conductivity, are critical for nearly all applications of multifunctional perovskite oxide ferroelectrics. Here we analysed possibility to induce semiconductor behaviour in these materials, which are basically insulators, by replacement of several percent of oxygen atoms with nitrogen, hydrogen, or vacancies. We explored this approach for one of the best studied members of the large family of ABO3 perovskite ferroelectrics - strontium titanate (SrTiO3). The atomic and electronic structure of defects were theoretically investigated using the large-scale first-principles calculations for both bulk crystal and thin films. The results of calculations were experimentally verified by studies of the optical properties at photon energies from 25 meV to 8.8 eV for in-situ prepared thin films. It was demonstrated that substitutions and vacancies prefer locations at surfaces or phase boundaries over those inside crystallites. At the same time, local states in the bandgap can be produced by vacancies located both inside the crystals and at the surface, but by nitrogen substitution only inside crystals. Wide-bandgap insulator phases were evidenced for all defects. Compared to pure SrTiO3 films, bandgap widening due to defects was theoretically predicted and experimentally detected.

First-principles calculations of oxygen octahedral distortions in LaAlO3/SrTiO3(001) superlattices

The size, shape and connectivity of oxide octahedra are essential for understanding and controlling the emergent functional properties of ABO₃ perovskites. Using first-principles calculations, we systematically studied the oxygen octahedral rotation and deformation in LaAlO₃/SrTiO₃(001) superlattices. Superlattices with electron- or hole-doped interfaces, or both, are compared. The results showed that there are at least three different types of oxygen octahedral distortions in these superlattices, which is more than what had previously been reported in the literature. We demonstrate that interfacial oxygen octahedral coupling and hole-doping, in addition to epitaxial strain, are the key factors underlying the formation of multiple types of oxygen octahedral rotations in these systems. We confirm that oxygen octahedral rotations and deformations play an essential role in insulator-metal transitions. Furthermore, octahedral distortion leads to ferroelectricity like dipole formation with the polarization vector always pointing to the positively charged interfaces.