Theory of structural response to macroscopic electric fields in ferroelectric systems

TL;DR

This paper develops a formalism within density-functional perturbation theory to compute how ferroelectric structures respond to static electric fields, enabling analysis of phase transitions and dielectric properties.

Contribution

It introduces a novel DFPT-based approach to determine equilibrium structures and properties of ferroelectrics under electric fields, incorporating polarization constraints and Berry-phase derivatives.

Findings

Analyzed field-induced structural phase transitions in ferroelectrics.

Computed linear and nonlinear dielectric constants including lattice contributions.

Demonstrated the method on BaTiO3 and PbTiO3 structures.

Abstract

We have developed and implemented a formalism for computing the structural response of a periodic insulating system to a homogeneous static electric field within density-functional perturbation theory (DFPT). We consider the thermodynamic potentials E(R,eta,e) and F(R,eta,e) whose minimization with respect to the internal structural parameters R and unit cell strain eta yields the equilibrium structure at fixed electric field e and polarization P, respectively. First-order expansion of E(R,eta,e) in e leads to a useful approximation in which R(P) and eta(P) can be obtained by simply minimizing the zero-field internal energy with respect to structural coordinates subject to the constraint of a fixed spontaneous polarization P. To facilitate this minimization, we formulate a modified DFPT scheme such that the computed derivatives of the polarization are consistent with the discretized form of the Berry-phase expression. We then describe the application of this approach to several problems associated with bulk and short-period superlattice structures of ferroelectric materials such as BaTiO3 and PbTiO3. These include the effects of compositionally broken inversion symmetry, the equilibrium structure for high values of polarization, field-induced structural phase transitions, and the lattice contributions to the linear and the non-linear dielectric constants.