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

Abstract

We present a rationalization of the Raman spectra of orthorhombic and rhombohedral, stoichiometric and doped, manganese perovskites. In particular, we study $R{\mathrm{MnO}}_{3}$ $(R=\mathrm{La},$ Pr, Nd, Tb, Ho, Er, Y, and Ca) and the different phases of Ca- or Sr-doped $R{\mathrm{MnO}}_{3}$ compounds as well as cation deficient $R{\mathrm{MnO}}_{3}.$ The spectra of manganites can be understood as combinations of two kinds of spectra corresponding to two structural configurations of ${\mathrm{MnO}}_{6}$ octahedra and independently of the average structure obtained by diffraction techniques. One type of spectra corresponds to the orthorhombic $\mathrm{Pbnm}$ space group for octahedra with cooperative or dynamic Jahn-Teller distortions, with stretching modes as the main features and whose frequencies correlate to Mn-O distances. The other spectrum is associated to regular but tilted octahedra whose modes can be described in the rhombohedral $R\overline{3}c$ structure, where only bending and tilt modes are observed. The main peaks of compounds with regular ${\mathrm{MnO}}_{6}$ octahedra, such as ${\mathrm{CaMnO}}_{3},$ highly Ca-doped ${\mathrm{LaMnO}}_{3},$ or the metallic phases of Ca- or Sr-doped ${\mathrm{LaMnO}}_{3},$ are bending and tilt ${\mathrm{MnO}}_{6}$ octahedra modes which correlate to $\mathrm{R}\ensuremath{-}\mathrm{O}(1)$ 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 microphases or macrophases present simultaneously in many manganites.