Electronic state of the ⁵⁷Fe probe atoms in perovskites LaMO₃ (M = Ni, Cu)

Abstract

Mössbauer spectroscopy has been applied for comparative study of the electronic state of probe iron atoms in perovskite-like nikelate LaNi_0.99 ⁵⁷Fe_0.01O₃ and cuprate LaCu_0.99 ⁵⁷Fe_0.01O₃. The valence states of the ⁵⁷Fe atoms in these isostructural matrices are significantly different. In the nikelate, the iron atoms are in one of its ordinary valence state Fe³⁺, whereas, in the cuprate, the iron atoms have the formal oxidation state +4, as follows from hyperfine coupling parameters. The observed differences between the valence states of the ⁵⁷Fe probe atoms are attributed to the different electronic structures of the Ni and Cu atoms in the perovskites. Configuration interaction calculations show that the major contribution to the electronic state of the cation-anion complex [NiO₆]⁹⁻ in the nikelate is made by the d ⁷ configuration. For the cuprate, the d ⁹ $\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle-}$}}{L}$ configuration dominates the electronic state of the [CuO₆]⁹⁻ complex. The existence of one electron hole ($\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle-}$}}{L}$) on oxygen atoms leads to partial charge transfer from the doped iron atoms in the cuprate, Fe³⁺ (d ⁵) + O⁻ ($\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle-}$}}{L}$) → Fe⁴⁺ (d ⁴) + O²⁻. As a result, the ground state of the dopant cation-anion complex FeO₆ can be represented as a superposition of configurations, d ⁴ (55%) and d ⁵ $\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle-}$}}{L}$ (45%).