Small Polarons and Bipolarons

We report on far- and mid-infrared emission and reflectivity from 40 cm^-1 to 13000 cm^-1 of ambient orthorhombic O-ErMnO₃ to sample dissociation at ~1800 K. While at 300 K the number of phonons are in close agreement with the predicted for D_2h¹⁶(Z=4), it gradually decreases by progressive broadening, merging, and band softening into the orbital disordered phase in which the orthorhombic O' lower temperature cooperative coexists with orthorhombic O. Reik’s small polaron[1] fits of the optical conductivities in the Jahn-Teller cooperative phase and in the orbital disordered phase below 1250 K single out the octahedral symmetric and antisymmetric as main phonons in the electron-phonon interactions. At higher than the order-disorder transition at T_cubic ~1329 K ± 10 K three infrared active F_1u modes coincide with the expected for perovskite Pm-3m (Z=1). A distinctive band peaking at ~6000 cm^-1 is associated to small bipolarons (two electrons self-trapped in a bonding connecting orbital mediated by the longitudinal mode distortion). At ~1700 K its optical conductivity only involves the cubic highest longitudinal optical mode. An overdamped Drude term by itinerant carriers coexisting with localized bipolarons is found at the highest temperatures.[1] H. G. Reik, Z. Phys. 203,346 (1967).