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We report on the high-temperature evolution of far- and mid- infraredreflectivity and emissivity spectra of ambient orthorhombic ErMnO3 from 12 K tosample decomposition above 1800 K. At low temperatures the number of phononsagrees with the predictions for orthorhombic space group D2h16-Pbnm (Z=4) andcoexists with a paramagnon spin resonance and rare earth crystal fieldtransitions. Increasing the temperature, a number of vibrational bands undergoprofile broadening and softening approaching the orbital disordered phase wherethe orthorhombic O' lower temperature cooperative phase coexists withcubic-orthorhombic O. O-ErMnO3 undergoes a first order order-disordertransition into the perovskite cubic phase at Tcubic ~1329 K where the threetriple degenerate phonons allowed by the space group Pm-3m(Z=1) are identified.At about 800 K, a quantitative small polaron analysis of the orthorhombicmid-infrared real part optical conductivity shows that antisymmetric andsymmetric breathing modes sustain the strongest electron-phonon interactions.Above Tcubic the bipolaron fingerprint profile is the mid-infrared dominant andonly feature. Its appearance correlates with the localized screening of thehighest vibrational mode reststrahlen band. We propose that the longitudinaloptical mode macroscopic field screening is consequence of dynamically sharing{\delta} disproportioned eg electrons hovering over the JT distorted octahedraldimer [Mn (QJT)3+{\delta} (Mn(QJT)3-{\delta}))O6/2]2. A thermal driveninsulator-metal transition is detected with onset ~1600 K. We also address theoccurrence of an inhomogeneity induced THz band result of heating the samplesin dry air, triggering Mn3+-Mn4+ double exchange, under the presence of Mn4+smaller ions stabilizing the orthorhombic lattice.

Far- and Mid-infrared Emission and Reflectivity of Orthorhombic and Cubic ErMnO3: polarons and bipolarons