Sr ₃Fe _5/4Mo _3/4O _6.9, an n = 2 ruddlesden-popper phase: Synthesis and properties

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.