Ab initio linear response and frozen phonons for the relaxor PMN (PbMg1/3Nb2/3O3)

TL;DR

This study uses first-principles density functional methods to analyze phonon modes, dielectric properties, and structural stability of PMN relaxor ferroelectrics, revealing mode couplings and polarization pathways.

Contribution

It provides detailed ab initio insights into phonon instabilities, mode couplings, and the anisotropic Born charges in PMN, advancing understanding of its electromechanical properties.

Findings

Polar phonon modes show significant coupling with electric fields.

A polar TO mode is unstable in the ferroelectric phase.

Multiple polar mode instabilities suggest polarization rotation pathways.

Abstract

We report first principles density functional studies using plane wave basis sets and pseudopotentials and all electron linear augmented plane wave (LAPW) of the relative stability of various ferroelectric and antiferroelectric supercells of PMN for 1:2 chemical ordering along [111] and [001]. We used linear response with density functional perturbation theory (DFPT) as implemented in the code ABINIT to compute the Born effective charges, electronic dielectric tensors, long wavelength phonon frequencies and LO-TO splittings. The polar response is different for supercells ordered along [111] and [001]. Several polar phonon modes show significant coupling with the macroscopic electric field giving giant LO-TO splittings. For [111] ordering, a polar transverse optic (TO) mode with E symmetry is found to be unstable in the ferroelectric P3m1 structure and the ground state is found to be triclinic. Multiple phonon instabilities of polar modes and their mode couplings provide the pathway for polarization rotation. The Born effective charges in PMN are highly anisotropic and this anisotropy contributes to the observed huge electromechanical coupling in PMN solid solutions.