College Park, Maryland      June 6 - 10 , 2004

T3-A2 (4:15 PM): Charge Ordering, Long and Short Range Magnetic Ordering in the Mn3+/Mn4+ Oxide, Pb3Mn7O15.

J.E. Greedan, J. Barbier, J.F. Britten, N. Penin, H. Ghazi (BIMR and Chemistry Dept. ,McMaster University, Hamilton CANADA), C.R. Wiebe (Department of Physics and Astronomy, McMaster University, Hamilton, ON, Canada), H. Dabkowska (BIMR, McMaster University, Hamilton,CANADA), I.P. Swainson (National Research Council, Chalk River, Ontario, Canada)

Manganate Mn3+/Mn4+ oxides have been of intense interest in recent years. In particular the perovskite based materials, A1-xLnxO3( A = divalent alkaline earth, Ln = trivalent lanthanide) exhibit charge ordering, a variety of magnetic phenomena,metal insulator transitions and of course, CMR. As well, the spinel LiMn2Mn4O4 shows partial charge ordering, complex long range and short range ordering and is of interest as a battery cathode. Pb3Mn7O15 is an under-investigated manganate with approximately equal concentrations of Mn3+ and Mn4+ and is structurally related to spinel in that the Mn-O octahedra are linked by edge-sharing. The compound forms in an orthorhombic structure (Cmcm) with pronounced hexagonal pseudo-symmetry. Neutron powder diffraction was instrumental in confirming the Cmcm structure at room temperature and that the charge ordering is only partial. The Mn sublattice is based on layers of edge-sharing triangles and is thus prone to geometric frustration. This is reflected in the magnetic susceptibility that shows an extended region of short range order from above 350 K to 72 K followed by magnetic transitions at 72 K and 55 K. The magnetic structure between 72 K and 55 K is described by k = (000) while a larger cell, k = (1/2 1/2 0), sets in below 55 K. Neutron diffuse scattering is clearly seen in the form of a broad Lorentzian background centered at Q = 1.36 -1 which is observed over the range from ~ 60 K (below the first magnetic transition) to at least 180 K. The correlation length derived from the Lorentzian peak width is ~ 4.6 which is three Mn neighbor distances, slightly longer than found for LiMn2O4.

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