title: 15R SrMn(1-)(x)()Fe(x)()O(3)(-)(delta) (x approximately 0.1); A New Perovskite Stacking Sequence.
abstract: A polycrystalline sample of a new phase in the Sr-Fe-Mn-O system has been prepared by standard solid-state techniques. Characterization at room temperature by X-ray diffraction, high-resolution electron microscopy, Mössbauer spectroscopy and neutron diffraction has led to it being described as a 15-layered, rhombohedral (15R) perovskite [space group R&thremacr;m: a = 5.4489(1) Å, c = 33.8036(7) Å] with a previously unobserved structure. The pseudo close-packed SrO(3) layers have a (cchch)(3) stacking sequence such that the occupation of the interstitial 6-coordinate sites by Mn (or Fe) leads to the formation of Mn(2)O(9) units which are linked to each other either directly by a common vertex, or indirectly via a single, vertex-sharing MnO(6) octahedron. The stoichiometry of the compound was determined to be SrMn(0.915(5))Fe(0.085(5))O(2.979(3)). The face-sharing sites are occupied by 0.957(3)Mn/0.043(3)Fe while the exclusively corner-linked sites show a higher Fe occupation; 0.745(4)Mn/ 0.255(4)Fe. A neutron diffraction experiment carried out at 3 K indicated the presence of long-range magnetic order with the Mn(4+) cations aligned antiferromagnetically with an ordered moment of 2.26(3)&mgr;(B)/Mn(4+). Both the neutron and the susceptibility data are consistent with the Fe cations remaining magnetically disordered to 3 K. The latter data show T(N) = 220 K, and suggest that some spin frustration is present at low temperatures.
authors.affiliation: Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QR, U.K., Department of Materials, University of Oxford, Parks Road, Oxford OX1 3QR, U.K., and School of Chemistry, Leeds University, Leeds LS2 9JT, U.K.
authors.forename: Edmund J.
authors.initials: EJ
authors.lastname: Cussen
authors.forename: Jeremy
authors.initials: J
authors.lastname: Sloan
authors.forename: Jaap F.
authors.initials: JF
authors.lastname: Vente
authors.forename: Peter D.
authors.initials: PD
authors.lastname: Battle
authors.forename: Terence C.
authors.initials: TC
authors.lastname: Gibb