ISSN 1063-7834, Physics of the Solid State, 2014, Vol. 56, No. 12, pp. 2519–2523. © Pleiades Publishing, Ltd., 2014. Original Russian Text © A.P. Chernyshev, S.A. Petrov, N.F. Uvarov, 2014, published in Fizika Tverdogo Tela, 2014, Vol. 56, No. 12, pp. 2430–2434. 2519 1. INTRODUCTION Perovskites belong to a rare class of compounds in which iron has an unusual oxidation state. For exam- ple, iron ions in SrFeO 3 have the oxidation state 4+ and are in high-spin state . Here, is consid- ered as a doubly degenerated narrow energy band formed by e g orbitals of Fe 4+ ions. These ions have a symmetric electron configuration; therefore, the octa- hedra could be distorted due to the Jahn–Teller effect. However, the distortion is not observed, because elec- tron is delocalized [1]. Compound SrFeO 3 has the cubic symmetry, exhibits metallic conductivity, and is not prone to the charge disproportionation. This com- pound retains the metallic conductivity and the cubic symmetry to 4 K. However, when a part of Sr 2+ anions in this compound is replaced by rare-earth ions, the obtained ferrite with a mixed valence demonstrates the charge disproportionation in the antiferromagnetic phase. The charge disproportionation in La 1/3 Sr 2/3 FeO 3– δ , where iron is formally represented as 67% Fe 4+ and 33% Fe 3+ , leads to the formation of a charge-ordered phase in which the iron state is con- ventionally represented as Fe 3+ Fe 3+ Fe 5+ . The exist- ence of the charge ordering at a temperature lower than ~200 K was established by Mössbauer spectros- copy [2, 3], neutron diffraction [4], and transmission electron microscopy [5]. At a temperature about 200 K, La 1/3 Sr 2/3 FeO 3– δ undergoes the transition to the paramagnetic phase with the average valence state, which is accompanied by a sharp (almost by an order of magnitude) decrease in the resistivity [6]. t 2 g 3 e g * 1 e g * e g * 1 The study of a series of compounds (SrFeO 3 , CaFeO 3 , and others) showed that their electrical and magnetic properties are determined by holes rather than by electrons [7, 8]. It was found that the electron configuration of the Fe–O octahedron is Fe 3+ (O 6 ) 11– . This configuration is often denoted as Fe 3+ L , where L means a hole belonging to the oxygen octahedron [7]. In our work, this configuration is denoted as [Fe 3+ – L ], which better reflects the character of the cation– ligand bond. It is generally agreed that, in the para- magnetic phase of La 1/3 Sr 2/3 FeO 3– δ , the iron cation has the formal valence 11/3 (Fe 11/3+ ) and can be con- sidered as SrFeO 3 with an added electron [9]. In SrFeO 3 , oxygen holes L remain delocalized at any temperatures. In the Takano model [2], it is taken that oxygen holes in La 1/3 Sr 2/3 FeO 3– δ are localized upon cooling according to the following mechanism: [3Fe 3+ –2L ] (metallic conductivity, T ≥ 200 K) 2Fe 3+ + [Fe 3+ –2L ] (semiconductor, T < 200 K) [2, 4, 10, 11]. The stepwise change in the valence in the charge-ordered state is considered as the condensa- tion of the hole bipolaron and the formation of the [Fe 3+ –2L ] cation–bipolaron complex in a magnetic field of the sublattice consisting of Fe 3+ (3d 3 ) ions [2, 9]. However, the assumption used in the Takano model that a part of iron cations has oxidation state 5+ at temperatures lower than the phase transition temper- ature is not confirmed by the data on the chemical shift. The observed chemical shift, which is approxi- mately equal to 0.07 mm/s, corresponds to the oxida- tion state of iron cations 4+ [12, 13]. The chemical shift due to the existence of Fe 5+ in the perovskite structure should be from –0.5 to –0.3 mm/s [13]. The PHASE TRANSITIONS Mössbauer Spectroscopy Investigation of the La 1/3 Sr 2/3 FeO 3– δ Perovskite A. P. Chernyshev*, S. A. Petrov, and N. F. Uvarov Institute of Solid State Chemistry and Mechanochemistry, Siberian Branch of the Russian Academy of Sciences, ul. Kutateladze 18, Novosibirsk, 630128 Russia e-mail: alfred.chernyshev@solid.nsc.ru Received February 7, 2014 Abstract—The charge state of iron in the La 1/3 Sr 2/3 FeO 3– δ 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 dis- proportionation occurs according to the scheme 3Fe 11/3+ 2Fe 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+ –2L ] [2Fe 3+ –L ] + [Fe 3+ –L ]. Simultaneously, the energy gap is formed with the maximum width of 0.14 ± 0.02 eV determined by Mössbauer spectroscopy. DOI: 10.1134/S1063783414120075