One-center Charge Transfer Transitions in Manganites

Abstract

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 (formula presented)-Mn (formula presented) CT transitions in (formula presented) 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 (formula presented) and (formula presented) CT transitions. We present a self-consistent description of the CT bands in insulating stoichiometric (formula presented) compound with the only (formula presented) valent state and idealized octahedral (formula presented) 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 (formula presented) 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 (formula presented) and (formula presented) CT transitions. These predictions are in good agreement with experimental spectra. The experimental data points to a strong overscreening of the crystal field parameter (formula presented) in the CT states of (formula presented) centers. © 2002 The American Physical Society.