Charge dynamics of ⁵⁷Fe probe atoms in La₂Li_0.5Cu_0.5O₄

The objective of this study is to characterize the electronic state and local surrounding of ⁵⁷Fe Moessbauer - probe atoms within iron-doped layered perovskite La₂Li_0.5Cu_0.5O₄ containing transition metal in unusual formal oxidation states '³⁺'. An approach based on the qualitative energy diagrams analysis and the calculations within the cluster configuration interaction method have been developed. It was shown that a large amount of charge is transferred via Cu-O bonds from the O: 2p bands to the Cu: 3d orbitals and the ground state is dominated by the d⁹L configuration ('Cu²⁺-O^-' state). The dominant d⁹L ground state for the (CuO₆) sublattice induces in the environment of the ⁵⁷Fe probe cations a charge transfer Fe³⁺ O^-(L) / Fe⁴⁺ + O^2-, which transforms 'Fe³⁺' into 'Fe⁴⁺' state. The experimental spectra in the entire temperature range 77-300 K were described with the use of the stochastic two-level model based on the assumption of dynamic equilibrium between two Fe³⁺ -(hash)reversible-(hash) Fe⁴⁺ valence states related to the iron atom in the [Fe(1)O₄]^4- center. The relaxation frequencies and activation energies of the corresponding charge fluctuations were estimated based on Moessbauer data. The results are discussed assuming a temperature-induced change in the electronic state of the [CuO₄]^5- clusters in the layered perovskite. (authors)