DFT studies of BaTiO3 interfaces

Abstract

This thesis has explored some of the nuances of Density Funtional Theory, ferroelectricity and oxide perovskites, BaTiO$_3$ in particular. The effects of strain and crystal orientation on phase diagram and polarization in BTO has been investigated using DFT calculations and Berry phase analysis. There has been done DFT calculations on bulk BTO as well as epitaxially strained BTO, and by using Berry phase analysis the polarization magnitude and direction of the structures have been estimated.

The polarization of (001) strained BTO increase for both compressive and tensile strain as the favoured structure transition from a P4mm symmetry with the extraordinary axis out of the strain plane via a Cm symmetry to a Amm2 symmetry with ion displacements along the pseudocubic [110] direction. For the (111) strained samples the structure has a first order phase transition at 1.5\% tensile strain from a Cm symmetry with ion displacements in the out of plane [111] direction to a P1 symmetry. The results from the tetragonal phase is used to get an estimate of the behaviour at room temperature, as the tetragonal phase is favoured between 5$^{\circ}$C and 120$^{\circ}$C. This shows similar results as for the rhombohedral phase for (111) strained samples. The work done in this thesis lays the theoretical groundwork for a polar phase for compressive strain in (111) strained BTO, where all the polar phases converge to a polar out of plane phase for compressive strain using the PBEsol functional. This is contrary to some of the previous work done on (111) oriented BTO stating that BTO relaxes to a non-polar R-3m symmetry for compressive strain values where the LDA functional has been used.