H-ErMnO₃ electrodynamics from far-infrared emission, reflectivity, and absorption in the ferroelectric and paraelectric phases

We report on the temperature evolution of ambient non-centrosymmetric hexagonal H-ErMnO₃ -P6₃cm-(Z=6) from 2 K to decomposition as seen in absorption, near normal reflectivity, and emission spectra in the THz to mid-infrared spectral range. While up to 300 K our measurements are in agreement with known spectra, in the ferroelectric phase below T_C~833 K, there is a strong continuous change in phonon profiles involving the two non-equivalent Er sites and bipyramids tilting. Three weak but sharp lowest frequency-lower temperature hybrid modes also melt into the background at about Tc. In emission spectra, toward the paraelectric centrosymmetric (P6₃ /mmc, Z=2) phase, an optical conductivity mid-infrared intraband becomes distinctive. It peaks at ~5000 cm^-1 and has the energy and profile fingerprint of small polarons (bipolarons). Its intensity depends on electric dipole ordering. At 900 K, just above T_C, this pronounce band starts a continuous weakening shifting to lower frequencies up to the temperature triggering hopping conduction onset of decomposition. We propose that oxygen polarizability (partially delocalized 2p-electrons) is a main dynamical constituent for bipolarons in cooperative driven MnO₅ bipyramids and thus in spontaneous ferroelectric polarization.