2015: The metal-insulator transition was discovered in oxygen-deficient Fe-based perovskite oxide film at 620 K

MSU physicists (Leading research scientist Marina A. Andreeva, Dept. of Solid State Physics) in collaboration with the researchers from Kyoto Univ., Nagoya Inst. of Technology and Japan Synchrotron Radiation Research Inst. (SPring-8) discovered in oxygen-deficient Fe-based perovskite oxide film a metal-insulator transition and charge disproportionation for iron atoms by means of the synchrotron nuclear resonance reflectivity.

The films of oxygen-deficient Fe-based perovskite oxides, SrFeO_2.8 prepared on a (001) SrTiO₃ single-crystal substrate (made by pulsed laser deposition with further oxidation by the air-annealing at 773K) were investigated by X-ray diffraction with a conventional four-circle X-ray diffractometer equipped with a high-temperature sample stage, conductivity measurements in a voltage-source mode by a two-terminal method, by conversion electron Mössbauer spectroscopy at room temperature and by nuclear resonance reflectivity.

The measurements of the nuclear resonant scattering were performed in a total reflection geometry at the beam line BL09XU of SPring-8 at various temperatures. The main purpose was to determine the isomeric shifts of the Mössbauer transition at different temperatures which directly characterize the charge state of ⁵⁷Fe atoms in the sample. The theoretical analysis of the time spectra was made using the ‘'REFTIM’' program package, developed by M.A. Andreeva (Hyperfine Interact. 185, 17–21 (2008)).

As a result, it was discovered that the metal-insulator transition in SrFeO_2.8 thin film takes place at 620K. This temperature is much higher than for the bulk SrFeO_2.8 material (70K). Significant increase in 550K of the transition temperature is explained by the substrate-induced strains in the SrFeO_2.8 film. The observed transition at 620K is due to the charge disproportionation of Fe^3.66+ into Fe⁴⁺ and Fe³⁺, associated with oxygen-vacancy ordering. Oxygen coordination of transition metals is a key for functional properties of transition-metal oxides, because hybridization of transition-metal d- and oxygen p-orbitals determines correlations between charges, spins and lattices.

The results of this work have been published in the paper: K. Hirai, D. Kan, N. Ichikawa, K. Mibu, Y. Yoda, M. Andreeva, Y. Shimakawa, “Strain-Induced Significant Increase in Metal-Insulator Transition Temperature in Oxygen-Deficient Fe Oxide Epitaxial Thin Films”, Scientific Reports 5, 7894 (2015).