Lattice Relaxation and Charge-Transfer Optical Transitions Due to Self-Trapped Holes in Non-Stoichiometric LaMnO${}_3$ Crystal

Abstract

We use the Mott-Littleton approach to evaluate polarisation energies in LaMnO${}_3$ lattice associated with holes localized on both Mn${}^{3+}$ cation and O${}^{2-}$ anion. The full (electronic and ionic) lattice relaxation energy for a hole localized at the O-site is estimated as 2.4 eV which is appreciably greater than that of 0.8 eV for a hole localized at the Mn-site, indicating on the strong electron-phonon interaction in the former case. Using a Born-Haber cycle we examine thermal and optical energies of the hole formation associated with electron ionization from Mn${}^{3+}$, O${}^{2-}$ and La${}^{3+}$ ions in LaMnO${}_3$ lattice. For these calculations we derive a phenomenological value for the second electron affinity of oxygen in LaMnO${}_3$ lattice by matching the optical energies of La${}^{4+}$ and O${}^{-}$ hole formation with maxima of binding energies in the experimental photoemission spectra. The calculated thermal energies predict that the electronic hole is marginally more stable in the Mn${}^{4+}$ state in LaMnO${}_3$ host lattice, but the energy of a hole in the O${}^{-}$ state is only higher by a small amount, 0.75 eV, rather suggesting that both possibilities should be treated seriously. We examine the energies of a number of fundamental optical transitions, as well as those involving self-trapped holes of Mn${}^{4+}$ and O${}^{-}$ in LaMnO${}_3$ lattice. The reasonable agreement with experiment of our predicted energies, linewidths and oscillator strengths leads us to plausible assignments of the optical bands observed. We deduce that the optical band near 5 eV is associated with O(2p) - Mn(3d) transition of charge-transfer character, whereas the band near 2.3 eV is rather associated with the presence of Mn${}^{4+}$ and/or O${}^{-}$ self-trapped holes in non-stoichiometric LaMnO${}_3$ compound.

18 pages, 6 figures, it was presented partially at SCES-2001 conference in Ann Arbor, Michigan

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