First principles based atomistic modeling of phase stability in PMN-xPT

TL;DR

This study uses first-principles-based molecular dynamics simulations to explore phase stability and transitions in the relaxor-ferroelectric PMN-xPT across different compositions and temperatures.

Contribution

It introduces a shell model potential fitted to first-principles data to accurately simulate phase behavior in PMN-xPT, revealing detailed phase sequences and morphotropic boundaries.

Findings

PMN remains cubic and behaves as a polar glass at all temperatures.

Adding Ti induces a transition from polar glass to ferroelectric phases.

The phase diagram aligns with the random site model and shows clear phase sequences.

Abstract

We have performed molecular dynamics simulations using a shell model potential developed by fitting first principles results to describe the behavior of the relaxor-ferroelectric (1-x)PbMg1/3Nb2/3O3-xPbTiO3 (PMN-xPT) as function of concentration and temperature, using site occupancies within the random site model. In our simulations, PMN is cubic at all temperatures and behaves as a polar glass. As a small amount of Ti is added, a weak polar state develops, but structural disorder dominates, and the symmetry is rhombohedral. As more Ti is added the ground state is clearly polar and the system is ferroelectric, but with easy rotation of the polarization direction. In the high Ti content region, the solid solution adopts ferroelectric behavior similar to PT, with tetragonal symmetry. The ground state sequence with increasing Ti content is R-MB-O-MC-T. The high temperature phase is cubic at all compositions. Our simulations give the slope of the morphotropic phase boundaries, crucial for high temperature applications. We find that the phase diagram PMN-xPT can be understood within the random site model.