15R SrMn{sub 1{minus}x}Fe{sub x}O{sub 3{minus}{delta}} (x {approx} 0.1); A new perovskite stacking sequence

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, Moessbauer spectroscopy and neutron diffraction has led to it being described as a 15-layered, rhombohedral (15R) perovskite with a previously unobserved structure. The pseudo close-packed SrO{sub 3} layers have a (cchch){sub 3} stacking sequence such that the occupation of the interstitial 6-coordinate sites by Mn (or Fe) leads to the formation of Mn{sub 2}O{sub 9} units which are linked to each other either directly by a common vertex, or indirectly via a single, vertex-sharing MnO{sub 6} octahedron. The stoichiometry of the compound was determined to be SrMn{sub 0.915(5)}O{sub 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/09.255(4)Fe. A neutron diffraction experiment carried out at 3 K indicated the presence of long-range magnetic order with the Mn{sup 4+} cations aligned antiferromagnetically with an ordered moment of 2.26(3){mu}{sub B}/Mn{sup 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{sub N} = 220 K, and suggest that some spin frustrationmore » is present at low temperatures.« less