Development of the high-temperature phase of hexagonal manganites -: art. no. 134108

Reports about the ferroelectric ordering temperatures in the multiferroic hexagonal RMnO3 system are controversial: transition temperatures varying between approximate to900 K and approximate to1300 K are reported for the same material. To elucidate the structural changes leading to ferroelectric distortions in hexagonal manganites, we calculate the irreducible representations of the distortions from the possible high-temperature symmetry P6(3)/mmc to the low-temperature symmetry P6(3)cm. There are four different orthogonal modes, of which only one allows a spontaneous electric polarization. Structure refinements and an accurate statistical analysis of neutron powder-diffraction data of TmMnO3, based on this group-theoretical analysis, reveal two phase transitions: We extrapolate a polar to nonpolar transition temperature of T-npt=1433(27) K, where the hexagonal bitetrahedra start to tilt, while the ferroelectric distortion appears at T-FE=1050(50) K. For R=Lu, Yb the tilt of the bitetrahedra and the buckling of the R layers as well as the ferroelectric distortion were extrapolated to comparable temperatures.