One-Center Charge Transfer Transitions in Manganites

In frames of a rather conventional quantum-chemical approach, which combines the crystal field and the ligand field models we have addressed different charge transfer (CT) states and O2p −Mn3d CT transitions in MnO 6 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 π−π and σ−σ CT transitions. We present a self-consistent description of the CT bands in LaMnO3 which allows to 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 t1g(π) − eg 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 σ− σ and π− π 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 centers. PACS codes: 71.10.-w, 71.15.-m, 71.15.Fv, 78.20.Bh