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