Raman phonons as a probe of disorder, fluctuations and local structure in doped and undoped orthorhombic and rhombohedral manganites

TL;DR

This paper demonstrates how Raman spectroscopy can effectively probe local structural variations, disorder, and fluctuations in doped and undoped manganites, revealing coexistence of different phases and dynamic polaron effects.

Contribution

It provides a detailed interpretation of Raman spectra in manganites, linking spectral features to local structural configurations and dynamic fluctuations, advancing understanding of phase coexistence.

Findings

Raman spectra reflect local MnO6 octahedral configurations.

Broad bands correlate with polaron hopping and dynamic fluctuations.

Coexistence of paramagnetic and metallic phases confirmed by Raman analysis.

Abstract

We present a rationalization of the Raman spectra of orthorhombic and rhombohedral, stoichiometric and doped, manganese perovskites. In particular we study RMnO3 (R= La, Pr, Nd, Tb, Ho, Er, Y and Ca) and the different phases of Ca or Sr doped RMnO3 compounds as well as cation deficient RMnO3. The spectra of manganites can be understood as combinations of two kinds of spectra corresponding to two structural configurations of MnO6 octahedra and independently of the average structure obtained by diffraction techniques. The main peaks of compounds with regular MnO6 octahedra, as CaMnO3, highly Ca doped LaMnO3 or the metallic phases of Ca or Sr doped LaMnO3, are bending and tilt MnO6 octahedra modes which correlate to R-O(1) bonds and Mn-O-Mn angles respectively. In low and optimally doped manganites, the intensity and width of the broad bands are related to the amplitude of the dynamic fluctuations produced by polaron hopping in the paramagnetic insulating regime. The activation energy, which is proportional to the polaron binding energy, is the measure of this amplitude. This study permits to detect and confirm the coexistence, in several compounds, of a paramagnetic matrix with lattice polaron together with regions without dynamic or static octahedron distortions, identical to the ferromagnetic metallic phase. We show that Raman spectroscopy is an excellent tool to obtain information on the local structure of the different micro or macro-phases present simultaneously in many manganites.