Sr3Fe5/4Mo3/4O6.9, an n = 2 Ruddlesen-Popper Phase: Synthesis and Properties
In a systematic search for an oxygen-stoichiometric phase, Sr{sub 3}(FeMo)O{sub 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{sub 3}Fe{sub 5/4}Mo{sub 3/4}O{sub 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 Sr2FeMoO6 adopts tetragonal I4/mmm symmetry (a = b = 3.92449(5) Angstroms; c = 20.3423(3) Angstroms) with vacancies at the O(1) oxygen site and with a composition that refines to a nominal stoichiometry Sr{sub 3}Fe{sub 5/4}Mo{sub 3/4}O{sub 6.9}. The two-phase SPXD refinement includes Sr{sub 3}Fe{sub 5/4}Mo{sub 3/4}O{sub 6.9} (95.7%) and a double-perovskite (DP) intergrowth, Sr{sub 2}FeMoO{sub 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{sub m} = {radical}2a {approx} 5.548 Angstroms, c{sub m} = c {approx} 20.35 Angstroms, derived from PND data, displays a saturated moment of 2.17(1) {mu}{sub B} at 9 K and an asynchronous decrease of the in-plane component of the Fe/Mo moment ({mu}{sub xy}), with respect to the out-of-plane moment ({mu}{sub z}) upon increasing temperature from 9 K up to the Neel temperature, TN {approx} 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 (189 K < T < 210 K, and 250 K < T < 280 K), with a metallic-like upturn at {approx}280 K and a low room-temperature resistivity ({rho}{sub RT} {approx} 3 {mu}{Omega}{center_dot}cm). A small negative magnetoresistance is observed ({approx}2.5%) at 5 T near the ordering temperature ({approx}150 K). The temperature-dependent magnetic susceptibility shows an inflection between 125 and 150 K, consistent with the AFM ordering temperature ({approx}150 K) observed by PND. X-ray near-edge spectroscopy data are consistent with formal charges of Fe{sup 3+} and Mo{sup 5+}. These properties are comparable to those found for Sr3FeMoO7- in our and other laboratories, except for the surprisingly low {rho}{sub RT} and the asynchronous temperature-dependent changes of {mu}{sub xy} and {mu}{sub z} at higher ordering temperature, which are unique to Sr{sub 3}Fe{sub 5/4}Mo{sub 3/4}O{sub 6.9}.
- Research Organization:
- Brookhaven National Lab. (BNL), Upton, NY (United States). National Synchrotron Light Source
- Sponsoring Organization:
- Doe - Office Of Science
- DOE Contract Number:
- DE-AC02-98CH10886
- OSTI ID:
- 914007
- Report Number(s):
- BNL-78575-2007-JA; CMATEX; TRN: US0801468
- Journal Information:
- Chem. Mater., Vol. 18; ISSN 0897-4756
- Country of Publication:
- United States
- Language:
- English
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