Far- and mid-infrared emission and reflectivity of orthorhombic and cubic ErMnO3: Polarons and bipolarons

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~1329K±20K 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 midinfrared 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 midinfrared 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 a consequence of dynamically sharing δ disproportioned eg electrons hovering over the Jahn-Teller distorted octahedral dimer {Mn(QJT)3+δ[Mn(QJT)[3]−δ]O6/2}2. A thermal driven insulator-metal transition is detected with onset ~1600 K. We also address the occurrence of an inhomogeneity induced terahertz 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.