The High Temperature Phase Transitions of Hexagonal YMnO3

TL;DR

This study uses high-resolution neutron diffraction to analyze the structural phase transitions of hexagonal YMnO3 from room temperature up to 1403 K, clarifying the nature of its ferroelectric transition and secondary phase changes.

Contribution

It provides detailed experimental evidence confirming the primary antiferrodistortive transition and rules out the intermediate phase, advancing understanding of YMnO3's ferroelectric behavior.

Findings

Transition at ~1258 K involves unit cell tripling and symmetry change to P6_3cm.

The primary transition is driven by the K3 antiferrodistortive mode.

Evidence of an isosymmetric transition at ~920 K with polarization decrease.

Abstract

We report a detailed high-resolution powder neutron diffraction investigation of the structural behaviour of the multiferroic hexagonal polymorph of YMnO3 between room temperature and 1403 K. The study was aimed at resolving previous uncertainties regarding the nature of the paraelectric- ferroelectric transition and the possibilities of any secondary structural transitions. We observe a clear transition at 1258 +/- 14 K corresponding to a unit cell tripling and a change in space group from centrosymmetric P6_3/mmc to polar P6_3cm. Despite the fact that this symmetry permits ferroelectricity, our experimental data for this transition analysed in terms of symmetry-adapted displacement modes clearly supports previous theoretical analysis that the transition is driven primarily by the antiferrodistortive K3 mode. We therefore verify previous suggestions that YMnO3 is an improper ferrielectric. Furthermore, our data confirm that the previously suggested intermediate phase with space group P6_3/mcm does not occur. However, we do find evidence for an isosymmetric phase transition (i.e. P6_3cm to P6_3cm) at ~920 K which involves a sharp decrease in polarization. This secondary transition correlates well with several previous reports of anomalies in physical properties in this temperature region and may be related to Y-O hybridization.