High-Temperature Lattice-Dynamics Evolution of YMnO3 and YbMnO3
Résumé
The exact mechanism responsible for the ferroelectricity in hexagonal manganite has been the subject of intense debate. Whether ferroelectricity and ferroelastic order appear at the same temperature and the role of covalency in the ferroelectric order are still discussed. High-temperature phase transitions are here investigated through the prism of lattice-dynamics evolution with temperature. Comparison is made between the YMnO3 and YbMnO3 behavior using polarized Raman spectroscopy. While YMnO3 shows two phase transitions (isosymetric at about 900 K and ferroelastic at 1200 K) YbMnO3 presents no lattice instability up to 1350 K. Phonons involved in the lattice instability and the zone tripling transition are identified. Moreover, peculiar hardening on heating of some YbMnO3 phonons is revealed and discussed, which further highlights the dynamical difference between these two hexagonal manganites.