One-Center Charge Transfer Transitions in Manganites

Abstract

In frames 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 MnO${}_6^{9-}$ octahedra. The many-electron dipole transition matrix elements were calculated using the Racah algebra for the cubic point group. Simple "local" approximation allowed 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 LaMn${}^{3+}$O${}_3$ compound with the only Mn${}^{3+}$ valent state and idealized octahedral MnO${}_6^{9-}$ centers which allows to substantially correct the current interpretation of the optical spectra. Our analysis shows the multi-band 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 $4.6÷4.7$ eV 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 point to a strong overscreening of the crystal field parameter $Dq$ in the CT states of MnO${}_6^{9-}$ centers.

10 pages, 3 figures

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