The improper ferroelectricity in YMnO3 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 YMnO3 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 P63cm symmetry. On heating, both local and average structural analyses show striking anomalies from ∼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.
@article{arxiv.1707.09649,
title = {Unconventional order-disorder phase transition in improper ferroelectric hexagonal manganites},
author = {Sandra H. Skjærvø and Quintin Meier and Mikhail Feygenson and Nicola A. Spaldin and Simon J. L. Billinge and Emil S. Bozin and Sverre M. Selbach},
journal= {arXiv preprint arXiv:1707.09649},
year = {2019}
}