High-temperature phase transitions of hexagonal YMnO<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow/><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

We report a detailed high-resolution powder neutron diffraction investigation of the structural behavior of the multiferroic hexagonal polymorph of YMnO${}_{3}$ 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 $1258\ifmmode\pm\else\textpm\fi{}14$ K corresponding to a unit-cell tripling and a change in space group from centrosymmetric $P{6}_{3}/\mathit{mmc}$ to polar $P{6}_{3}\mathit{cm}$. 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 YMnO${}_{3}$ is an improper ferrielectric. Furthermore, our data confirm that the previously suggested intermediate phase with space group $P{6}_{3}/\mathit{mcm}$ does not occur. However, we do find evidence for an isosymmetric phase transition (i.e., $P{6}_{3}\mathit{cm}$ to $P{6}_{3}\mathit{cm}$) at $\ensuremath{\approx}920$ 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.

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