H-ErMnO3 absorbance, reflectivity, and emissivity in the terahertz to mid-infrared from 2 to 1700 K: Carrier screening, Fröhlich resonance, small polarons, and bipolarons

Abstract

We report the temperature-dependent THz to mid-infrared response of hexagonal-ErMnO3 using absorption, reflectivity, and emissivity techniques from 2 to 1700 K. At low temperatures, lowest frequency vibrational modes, which extend up the lock-in ferroelectric temperature, coexist with paramagnon excitations and are associated with well-defined crystal-field rare-earth pure magnetic replicas in an intriguing phonon-magnetic convergence enhancing the multiferroic character of h-ErMnO3. Increasing the temperature, a number of vibrational bands close to the space group predicted undergo profile broadening and softening. In particular, a distinctive set of bands in the 288-329-cm-1 (300 K) range has a component whose profile is carrier screened becoming nearly fully blurred in the intermediate phase between ∼830 and ∼1500 K. Below TC∼830 K this asymmetric band, having one component still partially screened, further splits as spin phonon interaction and the tripling of the unit cell take place revealing at TN∼79 K a delicate balance of long- and short-range interactions. Ambient Raman scattering brings up evidence of a Fröhlich resonance due to Coulomb interactions between carriers and the macroscopic field linked to the corresponding longitudinal optical-phonon mode. We found it is dynamically correlated to the hexagonal c-axis negative thermal expansion. Quantitative analyses of the mid-infrared (MIR) optical conductivity show that it also plays a role in small polarons and mediates in high-temperature bipolarones. Bipolaron profiles at high temperatures change as the sample opacity increases when at ∼900 K straight stripes turn curly toward complex vortex-antivortex domain patterns in the paraelectric phase. At still higher temperatures a low-frequency Drude contribution is triggered by electron hopping signaling an insulator-metal phase transition at ∼1600 K while the MIR response suggests coexistence between single small polarons and bipolarons. On closing, we draw a parallel with improper ferroelectrics sustaining a lattice incommensurate intermediate phase and unit-cell tripling. We argue that in the h-RMnO3 (R=rare earth, Y) family of compounds the intermediate phase be considered incommensurate with onset at TINC∼1500 K and ferroelectric lock-in at TC∼830 K delimiting this regime in h-ErMnO3.