High Temperature Far Infrared Dynamics of Orthorhombic NdMnO3: Emissivity and Reflectivity

TL;DR

This study investigates the high-temperature infrared properties of NdMnO3, revealing changes in phonon modes, electron-phonon interactions, and conductivity mechanisms from room temperature to decomposition, highlighting complex couplings in rare earth manganites.

Contribution

It provides detailed experimental data on the evolution of infrared active phonons and electronic properties of NdMnO3 across a wide temperature range, including orbital disorder transition.

Findings

Number of infrared active phonons decreases with temperature.

Transition from orthorhombic to cubic phase observed in phonon spectra.

Presence of polaron mid-infrared band and overdamped Drude component at high temperatures.

Abstract

We report on near normal far- and mid-infrared emission and reflectivity of NdMnO3 perovskite from room temperature to sample decomposition above 1800 K. At 300 K the number infrared active phonons is in close agreement with the 25 calculated for the orthorhombic D2h16-Pbnm (Z=4) space group. Their number gradually decreases as we approach the temperature of orbital disorder at ~1023 K where the orthorhombic O' lower temperature cooperative phase coexists with the cubic orthorhombic O. At above ~1200 K, the three infrared active phonons coincide with the expected for cubic Pm-3m (Z=1) in the high temperature insulating regime. Heating samples in dry air triggers double exchange conductivity by Mn3+ and Mn4+ ions and a small polaron mid-infrared band. Fits to the optical conductivity single out the octahedral antisymmetric and symmetric vibrational modes as main phonons in the electron-phonon interactions at 875 K. For 1745 K, it is enough to consider the symmetric stretching internal mode. An overdamped defect induced Drude component is clearly outlined at the highest temperatures. We conclude that Rare Earth manganites eg electrons are prone to spin, charge, orbital, and lattice couplings in an intrinsic orbital distorted perovskite lattice favoring embryonic low energy collective excitations.