Interplay of strain, polarization and magnetic ordering in complex oxides from first principles.

摘要

We study mechanisms of structural and magnetic phase transitions in crystalline oxides from first principles. The focus is on epitaxial stabilization in perovskites and on magnetoelastic coupling and frustration in spinels. These materials and phenomena are of great interest for basic science and have important roles to play in the design and discovery of new functional materials.;The effects of epitaxial strain on the structure of the perovskite oxide CaTiO3 are investigated. Particular attention is paid to the stabilization of a ferroelectric phase related to the polar instability found in previous first-principles studies of calcium titanate in the ideal cubic perovskite structure. At 1.5% strain, we find an epitaxial orientation transition between the ab-ePbnm phase, favoured for compressive strains, and the c-ePbnm phase. For larger tensile strains, a polar instability, which was hidden in the equilibrium bulk structure, develops in the c-ePbnm phase and an epitaxial-strain-induced ferroelectric phase is obtained with polarization along a [110] direction with respect to the primitive perovskite lattice vectors of the square substrate. A ferroelectric rhombohedral R3c phase, with a different combination of octahedral rotations, is also found to be competitive in energy for large tensile strains, and might be observable under the application of additional perturbations, such as a small degree of cation substitution.;We present an ongoing project to construct a first-principles effective Hamiltonian to investigate the transition from the high-temperature cubic phase to a low-temperature low-symmetry phase observed in the spinel structure oxides CdCr2O4 and ZnCr2O4. The local modes included in the expansion are the chromium displacements, distortions of the cadmium- or zinc-centred tetrahedra, and the homogeneous strain. The magnetostructural coupling of these degrees of freedom to the spins of the chromium ions is included in the effective Hamiltonian parametrization and first-principles determination using a symmetry analysis. The role of the magnetostructural coupling in the phase transition is analysed and discussed.