Mode coupling between nonpolar and polar phonons as the origin of improper ferroelectricity in hexagonal LuMnO3

Abstract

Currently, the most puzzling problem associated with the hexagonal LuMnO₃ (h-LMO) is a large temperature-gap between the structural phase transition to the polar P6₃cm phase at ∼1290 K and the emergence of the spontaneous polarization at a substantially reduced temperature, ∼750 K. Interestingly, this large temperature-gap is not limited to h-LMO but is a universal phenomenon valid for other rare-earth manganites. We have examined this important issue by exploiting first-principles calculations. It is shown that the structural phase transition to the polar P6₃cm phase from the nonpolar P6₃/mmc phase of h-LMO is mediated by the freezing-in of the zone-boundary K₃ phonon. However, the ferroelectric polarization remains at a negligibly small value until the amplitude of the K₃ phonon reaches a certain critical value above which the coupling of the polar Γ₂⁻ mode with the nonpolar K₃ mode is practically turned on. This coupling-induced polarization explains the observed temperature-gap in h-LMO as well as other rare-earth manganites.