Mössbauer spectroscopy investigation of the La$_{1/3}$Sr$_{2/3}$FeO$_{3-\delta}$ perovskite

The charge state of iron in the La$_{1/3}$Sr$_{2/3}$FeO$_{3-\delta}$ perovskite has been studied using Mössbauer spectroscopy. It has been found that, as the temperature decreases in the range of 200–162 K, the charge disproportionation occurs according to the scheme $^3$Fe$^{11/3+}$ $\to$ $^2$Fe$^{3.5+}$+Fe$^{4+}$ and the antiferromagnetic phase is formed. The charge disproportionation is due to condensation of hole bipolarons. Each of bipolarons is collectivized by a group of three ions. The collectivization occurs according to the scheme [3Fe$^{3+}$–2$\underline{L}$ ] $\to$ [2Fe$^{3+}$–$\underline{L}$ ] + [Fe$^{3+}$–$\underline{L}$ ]. Simultaneously, the energy gap is formed with the maximum width of 0.14 $\pm$ 0.02 eV determined by Mössbauer spectroscopy.