One-center charge transfer transitions in manganites

In the framework of a rather conventional cluster approach which combines the crystal field and the ligand field models we have considered different charge transfer (CT) states and O 2p-Mn 3d CT transitions in MnO69- octahedra. The many-electron dipole transition matrix elements were calculated using the Racah algebra for the cubic point group. Simple local approximation allowed us to calculate the relative intensity for all dipole-allowed pi-pi and sigma-sigma CT transitions. We present a self-consistent description of the CT bands in insulating stoichiometric LaMn3+O3 compound with the only Mn3+ valent state and idealized octahedral MnO69- centers which allows us to substantially correct the current interpretation of the optical spectra. Our analysis shows the multiband structure of the CT optical response with the weak low-energy edge at 1.7 eV, associated with forbidden t(1g)(pi)-e(g) transition and a series of the weak and strong dipole-allowed high-energy transitions starting from 2.5 and 4.5 eV, respectively, and extending up to nearly 11 eV. The most intensive features are associated with two strong composite bands near 4.6-4.7 and 8-9 eV, respectively, resulting from the superposition of the dipole-allowed sigma-sigma and pi-pi CT transitions. These predictions are in good agreement with experimental spectra. The experimental data points to a strong overscreening of the crystal field parameter Dq in the CT states of MnO69- centers.