Sr 3Fe 5/4Mo 3/4O 6.9, an n = 2 ruddlesden-popper phase: Synthesis and properties

Louis W. Whaley, Maxim V. Lobanov, Denis Sheptyakov, Mark Croft, Kandalam V. Ramanujachary, Samuel Lofland, Peter W. Stephens, Jae Hyuk Her, Gustaaf Van Tendeloo, Marta Rossell, Martha Greenblatt

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Abstract

In a systematic search for an oxygen-stoichiometric phase, Sr 3(FeMo)O 7, in a range of iron-to-molybdenum ratios greater than 1:1 that typically give phase mixtures, we have found an n = 2 Ruddlesden-Popper phase, Sr 3Fe 5/4Mo 3/4O 6.9, as supported by synchrotron powder X-ray diffraction (SPXD), high-resolution transmission electron microscopy (HREM), and powder neutron diffraction (PND) results. By SPXD, this oxygen-deficient, B-site disordered, two-dimensional analogue of Sr 2FeMoO 6 adopts tetragonal I4/mmm symmetry (a = b = 3.92449(5) Å; c = 20.3423(3) A) with vacancies at the O(1) oxygen site and with a composition that refines to a nominal stoichiometry Sr 3Fe 5/4Mo 3/4O 6.9. The two-phase SPXD refinement includes Sr 3Fe 5/4Mo 3/4O 6.9 (95.7%) and a double-perovskite (DP) intergrowth, Sr 2FeMoO 6 (4.3%), consistent with HREM studies in which DP intergrowths but no individual DP grains were found. The G-type antiferromagnetically (AFM)-ordered structure of the phase, with the magnetic cell a m = √2a ∼ 5.548 Å, c m = c ∼ 20.35 Å, derived from PND data, displays a saturated moment of 2.17(1) μ B at 9 K and an asynchronous decrease of the in-plane component of the Fe/Mo moment (μ xy), with respect to the out-of-plane moment (μ Z) upon increasing temperature from 9 K up to the Neel temperature, T N ∼ 150 K. No structural transitions were observed over the entire temperature range studied: from 1.5 to 500 K. The temperature-dependent resistivity is consistent with Efros-Shklovskii variable-range hopping, applicable to two ranges of temperature (189K < T < 210 K, and 250 K < T < 280 K), with a metallic-like upturn at ∼280 K and a low room-temperature resistivity (ρ RT ∼ 3 μΩ·cm). A small negative magnetoresistance is observed (∼2.5%) at 5 T near the ordering temperature (∼150 K). The temperature-dependent magnetic susceptibility shows an inflection between 125 and 150 K, consistent with the AFM ordering temperature (∼150 K) observed by PND. X-ray near-edge spectroscopy data are consistent with formal charges of Fe 3+ and Mo 5+. These properties are comparable to those found for Sr 3FeMoO 7-δ in our and other laboratories, except for the surprisingly low ρ RT and the asynchronous temperature-dependent changes of μ xy and μ z at higher ordering temperature, which are unique to Sr 3Fe 5/4Mo 3/4O 6.9.

Original languageEnglish (US)
Pages (from-to)3448-3457
Number of pages10
JournalChemistry of Materials
Volume18
Issue number15
DOIs
StatePublished - Jul 25 2006

All Science Journal Classification (ASJC) codes

  • Chemistry(all)
  • Chemical Engineering(all)
  • Materials Chemistry

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