High-temperature phase transitions of hexagonal YMnO₃

We report a detailed high-resolution powder neutron diffraction investigation of the structural behavior 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(3)cm. Despite the fact that this symmetry permits ferroelectricity, our experimental data for this transition (analyzed in terms of symmetry-adapted displacement modes) clearly support previous theoretical analysis that the transition is driven primarily by the antiferrodistortive K-3 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)/mcmdoes not occur. However, we do find evidence for an isosymmetric phase transition (i.e., P6(3)cm to P6(3)cm) at approximate to 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.