15R SRMN1-XFEXO3-DELTA (X-APPROXIMATE-TO-0.1) - A NEW PEROVSKITE STACKING-SEQUENCE

A polycrystalline sample of a new phase in the Sr-Fe-Mn-O system has b een prepared by standard solid-state techniques. Characterization at r oom temperature by X-ray diffraction, high-resolution electron microsc opy, Mossbauer spectroscopy and neutron diffraction has led to it bein g described as a 15-layered, rhombohedral (15R) perovskite [space grou p <R(3)over bar>m: a = 5.4489(1) Angstrom, c = 33.8036(7) Angstrom] wi th a previously unobserved structure. The pseudo close-packed SrO3 lay ers have a (cchch)(3) stacking sequence such that the occupation of th e interstitial 6-coordinate sites by Mn (or Fe) leads to the formation of Mn2O9 units which are linked to each other either directly by a co mmon vertex, or indirectly via a single, vertex-sharing MnO6 octahedro n. The stoichiometry of the compound was determined to be SrMn0.915(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 orde r with the Mn4+ cations aligned antiferromagnetically with an ordered moment of 2.26(3)mu(B)/Mn4+. Both the neutron and the susceptibility d ata are consistent with the Fe cations remaining magnetically disorder ed to 3 K. The latter data show T-N = 220 K, and suggest that some spi n frustration is present at low temperatures.