Torsional mode as order parameter in H-ErMnO₃: mode screening, Fröhlich resonance, bipolarons

While far infrared absorption, normal reflectivity, and emission spectra of hexagonal ErMnO₃ give a quantitative picture of phonon softening and merging on heating, the torsional mode at ~306 cm^-1 (300 K), associated to the negative thermal expansion and a c axis hybridization of O-2p with Er³⁺ levels, displays at lock-in Tc~830 K a distinctive infrared line shape anomaly and reststrahlen screening within the context of a ferroelectric instability inferred for a Mott-Hubbard insulator. Since the longitudinal optical (LO) mode macroscopic field screening implies phonon dynamic coupling to charge fluctuations in an electron correlated insulating polar environment we used the above the gap λ_exc= 355 nm Raman line to turn on the Fröhlich mechanism at macroscopic field frequencies at which the interaction between carriers and the Coulomb long range, responsible of the TO-LO split, takes place. We found a new Raman activated band that has its contour red shifted overlapping the infrared reflectivity minimum. It implies a univocal visualization of a huge electron-lattice interaction concomitant with finding small polaron correlations for which that phonon mediates in the formation of high temperature H-ErMnO₃ mid-infrared bipolarons below the metal-insulator transition at ~1600 K.