Study of static and dynamical properties of complex antiferroelectrics materials

The aim of this dissertation is the investigation of the static and dynamical properties of the complex antiferroelectric materials using Effective Hamiltonian method and First principles calculations. In chapter 3, a novel elemental interatomic coupling in perovskite materials which bilinearly couples the antiferroelectric displacements of cations with the rotations of the oxygen octahedra. This new coupling explains a very complex crystal structure of prototypical antiferroelectric PbZrO3. My explanation provides a unified description of many other complex antipolar crystal structures in variety of perovskite materials, including the occurrence of incommensurate phases in some of them. In chapter 4, results and analysis of atomistic simulations explaining the dynamics of antiferroelectric distortions in BiFeO3 (BFO) bulk under hydrostatic pressure are reported. This system undergoes a phase transition on cooling from paraelectric Pm3 ̄m state at high temperatures to an intermediate P4/mbm phase followed by Pnma state at low temperatures. On the basis of my calculations, I prepared an analytical model of these phase transitions. The model developed can be easily applied to predict dynamics of antipolar cation motion in improper ferroelectrics. I found out that the antipolar modes do not soften themselves in the high temperature regime but they soften in the intermediate and Pnma phases, due to trilinear energetic coupling term. In chap- ter 5, the finite temperature behavior of the polar, antipolar, and antiferrodistortive phonons in a prototypical hybrid improper ferroelectric (BiFeO3)1/(NdFeO3)1 1:1 superlattice is studied. In the low-temperature phase, a spontaneous polarization appears due to trilinear coupling of structural and antiferroelectric (ferrroelectric) modes. In chapter 6, results of first-principles calculations to investigate and analyze properties of (001) thin films made of the most complex perovskite system, namely NaNbO3, are presented.Ferroelectric, antiferroelectric and antiferrodistortive properties of this thin film are reported and discussed, as a function of misfit strain