Charge Disproportionation and Antiferromagnetic Order of Sr ₃Fe ₂O ₇

Abstract

The charge disproportionation and the magnetic properties of Sr 3 Fe 2 O 7 , which has a doublelayered perovskite structure, have been investigated. From 57 Fe Mössbauer measurements, the Néel temperature T N and the critical temperature for the charge disproportionation reaction (2Fe 4+ → Fe 3+ + Fe 5+ ) are determined to be 120 K ± 2 K and 343 K ± 10 K, respectively. Above 343 K, spectra clearly show a Fe 4+ singlet. The 57 Fe Mössbauer spectrum at 4.5 K under externally applied longitudinal magnetic field consists of four magnetically split components and is similar to the spectrum obtained from CaFeO 3 with a helical spin structure. With an increase the external magnetic field, the relative intensity of the 2nd and 5th absorption peaks increases, which implies that the magnetic moments of Fe have a tendency to form a texture perpendicular to the direction of the external magnetic field.The crystal structure, magnetic and electronic properties of the high spin d 4 (t 3 2g e 1 g ) iron perovskites have been studied extensively in recent decades. SrFeO 3 , CaFeO 3 , Sr 2 FeO 4 and Sr 3 Fe 2 O 7 are typical perovskites with the high spin d 4 which display a wide variety of magnetic and electric properties. SrFeO 3 has a cubic perovskite structure, in which the angle of Fe-O-Fe is 180 • , and does not show any lattice distortion down to 4.2 K, showing a metallic-like conductivity. 1) SrFeO 3 shows an antiferromagnetic order with a helical spin structure below 127 K. Fe spins align parallel in (111) and tilt about 40 • between neighboring (111) planes. 2) The strong covalency of σ * [e g − O(2s, 2p σ ) − e g ] interactions in SrFeO 3 leads to the formation of a band and electron delocalization. 3) CaFeO 3 , with a distorted perovskite structure in which the angle of Fe-O-Fe is about 160 • , shows a charge disproportionation (CD) reaction (2Fe 4+ → Fe 3+ + Fe 5+ ) below 290 K due to a conduction band narrower than that of SrFeO 3 . 4) The 57 Fe Mössbauer spectrum obtained from CaFeO 3 below 290 K indicates that the iron ions occupy two nonequivalent sites as Fe 3+ and Fe 5+ , which show different hyperfine interaction parameters. 5, 6) The 57 Fe Mössbauer spectrum of cubic SrFeO 3 shows only one sextet at 4.2 K. 7) Sr 2 FeO 4 has a K 2 NiF 4 -type structure consisting of isolated FeO 6 octahedra sheets, and displays no CD. 8) Sr 3 Fe 2 O 7 consists of isolated double sheets of FeO 6 octahedra, which is called a layered perovskite structure. Sr 3 Fe 2 O 7 is antiferromagnetic below T N = 120 K and shows a CD reaction, 8,9) although the critical temperature of CD has not yet been determined precisely. The 57 Fe Mössbauer spectrum of Sr 3 Fe 2 O 6.92 at 295 K is reported to be a superposition of Fe 4+ and Fe

Magnetic phase diagram of Sr3Fe2O7−δ

“…For 7 − δ = 7.00, the resistivity anomalies coincide with the critical temperatures for charge disproportionation (T D ) and magnetic ordering extracted from earlier Mößbauer data 7 and from the magnetization and neutron scattering data reported above, respectively. Although the temperature range of our data above the chargedisproportionation transition is limited, the substantial resistivity upturn below this temperature indicates that the weakly-insulating character is a consequence of the correlation-driven charge disproportionation instability, rather than disorder.…”

57Fe and 151Eu Mössbauer study of charge disproportionation and magnetic properties in Eu1/3Sr2/3FeO3

“…The iron perovskite-type oxides with rare earth and alkaline earth atoms are interesting because of their unique properties such as charge disproportionation(CD) [1][2][3][4][5][6][7][8], charge and orbital ordering [2,4,9,10] and cluster glass behaviors [11][12][13]. The first example of CD transition in perovskite was CaFeO 3 at 290 K at which the Fe 4+ discrete as Fe 3+ and Fe 5+ , accompanied by a structural transition from Pbnm in the normal state to P2/n [6].…”

Enhancement of Oxygen Evolution Activity of Ruddlesden-Popper-Type Strontium Ferrite by Stabilizing Fe4<sup>+</sup>

“…As described in the introduction, Fe 4+ is considered to play an important role in the OER on Fe-based catalysts; however, Fe 4+ is unstable against CD and rapidly disappears in usual [26] Figure 4(a) and Figure 4(b), a higher current density was observed at 70˚C.…”