LATTICE RELAXATION AND CHARGE-TRANSFER OPTICAL TRANSITIONS DUE TO SELF-TRAPPED HOLES IN NONSTOICHIOMETRIC LaMnO₃ CRYSTAL
Kovaleva N.N., Gavartin J.L., Shluger A.L., Boris A.V., Stoneham A.M.

Received: April 12, 2001

PACS: 75.30.Vn, 71.55.Ht, 78.40.Ha

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We explore the role of electronic and ionic polarization energies in the physics of the «colossal» magnetoresistive (CMR) materials. We use the Mott-Littleton approach to evaluate polarization energies in the LaMnO₃ lattice associated with holes localized on both the Mn³⁺ 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 a strong electron-phonon interaction in the former case. The ionic relaxation around the localized holes differs for the anion and cation holes. The relaxation associated with Mn⁴⁺ is approximately isotropic, whereas ionic displacements around O^- holes show axial symmetry with the axis directed towards the apical oxygens. Using the Born-Haber cycle, we examine thermal and optical energies of the hole formation associated with the electron ionization from Mn³⁺, O^2-, and La³⁺ ions in the LaMnO₃ lattice. For these calculations, we derive a phenomenological value for the second electron affinity of oxygen in the LaMnO₃ lattice by matching the optical energies of the La⁴⁺ 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 Mn⁴⁺ state in the LaMnO₃ host lattice, but the energy of a hole in the O^- state is only higher by a small amount, 0.75 eV, 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 Mn⁴⁺ and O^- in the LaMnO₃ lattice. The reasonable agreement of our predicted energies, linewidths, and oscillator strengths with experimental data leads us to plausible assignments of the optical bands observed. We deduce that the optical band near 5 eV is associated with the O(2p)-Mn(3d) transition of a charge-transfer character, whereas the band near 2.3 eV is rather associated with the presence of Mn⁴⁺ and/or O^- self-trapped holes in the nonstoichiometric LaMnO₃ compound.