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 K 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 K 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 takes 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 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.

Crystal growth and spontaneous polarization mechanism of polar metal borate halides: Mn3B7O13Cl

“…According to previous studies, the resonance absorption in the 0.4–0.8 THz range represents the polarization of the [MnCl 2 O 4 ] unit. 30 In our experiments, we observed that within the temperature range of 180–320 K, the resonance absorption of [MnCl 2 O 4 ] gradually increases with temperature, but the magnitude of the increase is small. The enhancement of resonance absorption corresponds to a weakening of polarization intensity, consistent with our computational results (Fig.…”

Magnetoelastic properties of multiferroic hexagonal ErMnO3

“…The functional property of ErMnO3 that has attracted attention as having greatest potential for use in device applications is the electrical conductivity of domain walls [4,29,31,33,49,[67][68][69][70][71]. For some of the aspirations in this context to be achieved, it will be necessary to engineer domain walls into preferred configurations and then constrain the chosen microstructure to remain stable during long term operation of the device.…”

“…Members of the family of multiferroic hexagonal manganites, RMnO3 (R = Sc, Y, In, Dy-Lu), are ferroelectric due to a structural phase transition within the interval ~1000-1500 K [1][2][3][4][5] and antiferromagnetic due to a magnetic ordering transition within the interval ~60-130 K [6][7][8][9][10]. Their structures at high temperatures and at room temperature are in space groups P63/mmc and P63cm, respectively [3,5,[10][11][12][13].…”