Ab initio study of proper topological ferroelectricity in layered perovskite La2Ti2O7

TL;DR

This study uses first-principles calculations to reveal how layered La2Ti2O7 exhibits high-temperature ferroelectricity driven by oxygen octahedral rotations, highlighting the role of lattice topology in inducing polarization.

Contribution

It demonstrates that oxygen octahedral rotations in layered La2Ti2O7 induce ferroelectricity, showing how natural layering enables polar distortions typically non-polar.

Findings

Ferroelectric transition involves two strongly coupled soft modes.

Oxygen octahedral rotations induce spontaneous polarization.

Layered topology enhances polar character of structural distortions.

Abstract

We present a first-principles investigation of ferroelectricity in layered perovskite oxide La2Ti2O7 (LTO), one of the compounds with highest Curie temperature known (1770 K). Our calculations reveal that LTO's ferroelectric transition results from the condensation of two soft modes that have the same symmetry and are strongly coupled anharmonically. Further, the leading instability mode essentially consists of rotations of the oxygen octahedra that are the basic building block of the perovskite structure; remarkably, because of the particular topology of the lattice, such O6 rotations give raise to a spontaneous polarization in LTO. The effects discussed thus constitute an example of how nano-structuring -- provided here by the natural layering of LTO -- makes it possible to obtain a significant polar character in structural distortions that are typically non-polar. We discuss the implications of our findings as regards the design of novel multifunctional materials. Indeed, the observed proper ferroelectricity driven by O6 rotations provides the ideal conditions to obtain strong magnetoelectric effects.