Lattice-distortion couplings in antiferroelectric perovskite $\rm AgNbO_3$ and comparison with $\rm PbZrO_3$

TL;DR

This study uncovers the key lattice-distortion couplings in AgNbO3, revealing how anharmonic interactions stabilize its antiferroelectric phase and comparing these mechanisms with those in PbZrO3.

Contribution

It identifies the dominant anharmonic couplings responsible for stabilizing the AgNbO3 phase using symmetry analysis and first-principles calculations, highlighting differences from PbZrO3.

Findings

Oxygen octahedra rotations and cation antipolar motions are crucial for phase stabilization.

AgNbO3's primary distortion is octahedral rotation, with antipolar motions being secondary.

The stabilization mechanism involves hybrid-improper and triggered coupling effects.

Abstract

Lead-free antiferroelectric perovskite $\rm AgNbO_3$ is nowadays attracting extensive research interests due to its promising applications in energy storage. Although great progress has been made in optimizing the material performance, fundamental questions remain regarding the mechanism stabilizing the antiferroelectric $Pbcm$ phase. Here, combining structural symmetry analysis and first-principles calculations, we identified crucial anharmonic couplings of oxygen octahedra rotations and cation antipolar motions which contribute significantly to lowering the energy of the $Pbcm$ phase. The stabilization of this phase shows close similarities with the stabilization of the $Pbam$ phase in $\rm PbZrO_3$ except that in $\rm AgNbO_3$ the octahedra rotations are the primary distortions while the antipolar cation motions appear to be secondary. The appearance and significant amplitude of the latter are explained from the combination of hybrid-improper and triggered mechanisms.