Unconventional order-disorder phase transition in improper ferroelectric hexagonal manganites

TL;DR

This study uncovers an unconventional order-disorder phase transition in improper ferroelectric hexagonal manganites, revealing local symmetry fluctuations and a continuum of structures across the transition, using neutron scattering and first-principles calculations.

Contribution

It demonstrates the presence of a unique local symmetry lowering and a continuum of structures during the ferroelectric transition, advancing understanding of phase transitions in multiferroic materials.

Findings

Local and average structures are consistent at room temperature.

Anomalies in structure appear around 800 K, near the Curie temperature.

A continuum of structures emerges due to fluctuations, indicating an unconventional transition.

Abstract

The improper ferroelectricity in YMnO$_3$ and other related multiferroic hexagonal manganites are known to cause topologically protected ferroelectric domains that give rise to rich and diverse physical phenomena. The local structure and structural coherence across the ferroelectric transition, however, were previously not well understood. Here we reveal the evolution of the local structure with temperature in YMnO$_3$ using neutron total scattering techniques, and interpret them with the help of first-principles calculations. The results show that, at room temperature, the local and average structures are consistent with the established ferroelectric $P6_3cm$ symmetry. On heating, both local and average structural analyses show striking anomalies from $\sim 800$ K up to the Curie temperature consistent with increasing fluctuations of the order parameter angle. These fluctuations result in an unusual local symmetry lowering into a \textit{continuum of structures} on heating. This local symmetry breaking persists into the high-symmetry non-polar phase, constituting an unconventional type of order-disorder transition.