A re-examination of antiferroelectric PbZrO$_3$ and PbHfO$_3$: an 80-atom $Pnam$ structure

TL;DR

First principles DFT simulations reveal an 80-atom $Pnam$ structure as a likely low-temperature ground state for antiferroelectric PbZrO$_3$ and PbHfO$_3$, challenging previous assumptions about their $Pbam$ structures.

Contribution

The study identifies a new $Pnam$ structure as a more stable low-temperature phase for these materials, supported by cross-code reproducibility and electronic interaction treatments.

Findings

$Pnam$ structure is more stable than $Pbam$.

Dynamical instability in $Pbam$ leads to $Pnam$ formation.

Reproducibility across different DFT methods.

Abstract

First principles density functional theory (DFT) simulations of antiferroelectric (AFE) PbZrO$_3$ and PbHfO$_3$ reveal a dynamical instability in the phonon spectra of their purported low temperature $Pbam$ ground states. This instability doubles the $c$-axis of $Pbam$ and condenses five new small amplitude phonon modes giving rise to an 80-atom $Pnam$ structure. Compared with $Pbam$, the stability of this structure is slightly enhanced and highly reproducible as demonstrated through using different DFT codes and different treatments of electronic exchange & correlation interactions. This suggests that $Pnam$ is a new candidate for the low temperature ground state of both materials. With this finding, we bring parity between the AFE archetypes and recent observations of a very similar AFE phase in doped or electrostatically engineered BiFeO$_3$.