Properties of BaTiO3/BaZrO3 ferroelectric superlattices with competing instabilities

TL;DR

This study uses first-principles calculations to explore the ferroelectric and structural properties of BaTiO3/BaZrO3 superlattices, revealing quasi-two-dimensional ferroelectricity, complex ground states, and stress-induced polarization effects.

Contribution

It provides new insights into the interplay of ferroelectricity and structural instabilities in BaTiO3/BaZrO3 superlattices, including the role of phonon condensation and mechanical stress.

Findings

Quasi-two-dimensional ferroelectricity occurs in single-unit-cell BaTiO3 layers.

The ground state involves simultaneous condensation of polar phonons and octahedral rotations.

Mechanical stress relaxation influences the observed polarization component.

Abstract

Properties of (BaTiO$_3$)$_1$/(BaZrO$_3$)$_n$ ferroelectric superlattices (SLs) with $n = {}$1--7 grown in the [001] direction are calculated from first principles within the density functional theory. It is revealed that the quasi-two-dimensional ferroelectricity occurs in these SLs in the barium titanate layers with a thickness of one unit cell; the polarization is oriented in the layer plane and weakly interacts with the polarization in neighboring layers. The ferroelectric ordering energy and the height of the barrier separating different orientational states of polarization in these SLs are sufficiently large to provide the formation of an array of independent polarized planes at 300 K. The effect of the structural instability on the properties of SLs is considered. It is shown that the ground state is a result of simultaneous condensation of the $\Gamma_{15}$ polar phonon and phonons at the $M$ point (for SLs with even period) or at the $A$ point (for SLs with odd period); it is a polar structure with out-of-phase rotations of the octahedra in neighboring layers, in which highly polarized layers are spatially separated from the layers with strong rotations. The competition between the ferroelectric and structural instabilities in biaxially compressed SLs manifests itself in that the switching on of the octahedra rotations leads to an abrupt change of the polarization direction and can cause an improper ferroelectric phase transition to occur. It was shown that the experimentally observed $z$-component of polarization in the SLs can appear only as a result of the mechanical stress relaxation.