Systematic ab initio study of the phase diagram of epitaxially strained SrTiO_3

摘要

We use density-functional theory with the local-density approximation to study the structural and ferroelectric properties of SrTiO_3 under misfit strains. Both the antiferrodistortive (AFD) and ferroelectric (FE) instabilities are considered by calculating all the phases predicted by Pertsev et al. [Phys. Rev. B 61, R825 (2000)] based on the phenomenological Landau theory. The rotation of the oxygen octahedra and the movement of the atoms are fully relaxed within the constraint of a fixed in-plane lattice constant. We find a rich misfit strain induced phase transition sequence which is in overall agreement with the prediction by Pertsev et al. and is obtained only when the AFD distortion is taken into account. Nevertheless, the calculated locations of the phase boundaries are different from the prediction by Pertsev et al. We also find that compressive misfit strains induce ferroelectricity in the tetragonal low temperature phase only while tensile strains induce ferroelectricity in the orthorhombic phases only. The calculated FE polarization for both the tetragonal and orthorhombic phases increases monotonically with the magnitude of the strains. The AFD rotation angle of the oxygen octahedra in the tetragonal phase increases dramatically as the misfit strain goes from the tensile to compressive strain region while it decreases slightly in the orthorhombic (FO4) phase. This reveals why the polarization in the epitaxially strained SrTiO_3 would be larger when the tensile strain is applied, since the AFD distortion is found to reduce the FE instability and even to completely suppress it in the small strain region. Finally, our analysis of the average polar distortion and the charge density distribution suggests that both the Ti-O and Sr-O layers contribute significantly to the FE polarization.