Role of Jahn-Teller disorder in Raman scattering of mixed-valence manganites

Abstract

The mixed-valence perovskitelike manganites are characterized by the unique interrelation of Jahn-Teller distortions, electric and magnetic properties. The Jahn-Teller distortion follows the ${\mathrm{Mn}}^{3+}\ensuremath{\rightarrow}{\mathrm{Mn}}^{4+}$ charge transfer with some delay. Its development depends on the lifetime of Mn in the $3+$ state, governed by the ${\mathrm{Mn}}^{4+}/{\mathrm{Mn}}^{3+}$ ratio and magnetic correlations. The noncoherence of Jahn-Teller distortions in orthorhombic mixed-valence manganites and rhombohedral $R{\mathrm{MnO}}_{3}$ $(R=\mathrm{rare}$ earth) results in oxygen disorder. We demonstrate that the Raman spectra in this case are dominated by disorder-induced bands, reflecting the oxygen partial phonon density of states (PDOS). The PDOS origin of the main Raman bands in insulating phases of such compounds is evidenced by the similar line shape of experimental spectra and calculated smeared PDOS and disappearance of the PDOS bands in the ordered ferromagnetic metallic phase.