Electrical properties of improper ferroelectrics from first principles

TL;DR

This study uses first-principles calculations to analyze the structural and electrical properties of improper ferroelectrics, focusing on YMnO3 and PbTiO3/SrTiO3 superlattices, revealing how distortions influence ferroelectric behavior.

Contribution

It provides a first-principles perspective on improper ferroelectricity, explaining how structural distortions lead to polarization in different materials and guiding the design of multiferroic compounds.

Findings

YMnO3's polar state is robust due to oxygen polyhedra tilts.

In PTO/STO, antiferrodistortive instabilities do not induce polarity.

Boundary conditions influence the polar or non-polar states.

Abstract

We study the interplay of structural and polar distortions in hexagonal YMnO3 and short-period PbTiO3/SrTiO3 superlattices by means of first-principles calculations at constrained electric displacement field D. We find that in YMnO3 the tilts of the oxygen polyhedra produce a robustly polar ground state, which persists at any choice of the electrical boundary conditions. Conversely, in PTO/STO the antiferrodistortive instabilities alone do not break inversion symmetry, and open-circuit bundary conditions restore a non-polar state. We suggest that this qualitative difference naturally provides a route to rationalizing the concept of "improper ferroelectricity" from the point of view of first-principles theory. We discuss the implications of our arguments for the design of novel multiferroic materials with enhanced functionalities, and for the symmetry analysis of the phase transitions.