Evidence for valence fluctuation of Fe in Sr2FeMoO6−w double perovskite

Abstract

In this letter evidence for the formation of a valence-fluctuation state of iron, formally denoted as Fe2.5+, is presented. The system under study is the Sr2FeMoO6−w double perovskite, known for exhibiting a very large magnetoresistance. Samples of Sr2FeMoO6−w were synthesized by means of an encapsulation technique utilizing an Fe getter technique and characterized by Fe57 Mössbauer spectroscopy. From 5 K to room temperature the Mössbauer spectrum is dominated by a component with hyperfine parameter values between those expected for high-spin Fe3+ and high-spin Fe2+.

Iron valence in double-perovskite (Ba,Sr,Ca)2FeMoO6: isovalent substitution effect

“…2. The hyperfine parameters obtained from the computer fittings of the spectra are presented in Table 1 5,6 However, the actual isomer shift value of M1 depends on the choice of the A-site cation. The dependence is illustrated in Fig.…”

“…10 In terms of the magnitude of isomer shift a similar mixed-valence state had earlier been established by means of Mössbauer spectroscopy for the A-site ordered double-perovskite, SmBaFe 2 O 5+δ (δ ≈ 0). 11 In the case of SmBaFe 2 O 5+δ , the Fe II/III state separates upon cooling into equal amounts of divalent and trivalent iron at a Verweytype transition with T V ≈ 230 K, 11 while for Sr 2 FeMoO 6-w the mixed-valence state was confirmed to persist down to 5 K. 5 In terms of fine-tuning the charge distribution "isovalent" substitution at the A site is expected to provide us with interesting possibilities, as such a substitution keeps overall charge of the B cations constant, but is likely to influence the distribution of charge among the two cations. 3 (where R is a rare earth element and A an alkaline earth element) magnetoresistors with a simple perovskite structure it was found that with decreasing effective ionic radius at the (R,A)-cation site, the Curie temperature (T C ) decreased and the magnitude of the MR effect increased drastically.…”

Origin of neutron magnetic scattering in antisite-disorderedSr2FeMoO6double perovskites

“…If we define the parameter x as the fraction of Mo atoms at Fe positions, we have x=0.41 for the "disordered" sample and x=0. 16 for the "ordered" sample. Notice that x would take a value of 0.5 for a completely disordered sample.…”

Iron and molybdenum valences in double-perovskite (Sr,Nd)2FeMoO6: electron-doping effect

“…Mössbauer data revealed another type of iron species too, i.e. trivalent Fe atoms or antisite atoms (AS) sitting at the lattice site reserved for Mo in the completely ordered double-perovskite structure [2,6,7]. Also obtained was from Mössbauer measurements experimental evidence for antiphase boundaries (APB) in Sr 2 FeMoO 6 [8].…”

“…Initially an electronic structure based on localized 3d 5 , as seen by both 57 Fe Mössbauer [2] and Fe K-and L-edge XANES [3] spectroscopy probes.…”