College Park, Maryland      June 6 - 10 , 2004

WP12: The pillared perovskites La5Os3MO16(M=Mn, Co): Synthesis, crystal structure and magnetism

Lisheng Chi (National Research Council, Chalk River, Ontario, Canada; Department of Chemistry & Brockhouse Institute for Materials Research, McMaster University, Hamilton, Ontario L8S 4M1, Canada), Ian Swainson (National Research Council, Chalk River, Ontario, Canada), John E. Greedan (; Department of Chemistry & Brockhouse Institute for Materials Research,McMaster University, Hamilton, Ontario L8S 4M1, Canada)

Two new Os-containing compounds La5Os3MnO16, La5Os3CoO16 with pillared perovskite type structures have been synthesized by solid state reactions. Rietveld profile refinement revealed that both compounds crystallize in space group C-1 with cell parameters a=7.9648(9)┼, 7.9360(7)┼; b=8.062(1)┼, 8.0481(7)┼; c=10.156(2)┼, 10.0788(8)┼; a=90.25(1)░, 90.300(4)░;b=95.5(1)░, 95.340(4)░; g=89.95(2)░, 89.914(6)░ for La5Os3MnO16 and La5Os3CoO16 , respectively. The structure consists of corner-sharing, site-ordered octahedral layers of composition Os5+M2+O6, pillared by diamagnetic Os2O10 triply-bonded dimers and separated by > 10┼. SQUID measurements in the paramagnetic regime show nearly spin only effective moments for Os5+(S = 3/2) and the M2+ ions. La5Os3MnO16 has two magnetic transitions at 178K and 78K, corresponding to long range ordering and spin reorientation, respectively. Isothermal magnetization data are characteristic of meta-magnetic behavior. La5Os3CoO16 has a single magnetic transition at 36K. The magnetic structure of La5Os3MnO16 consists of ferrimagnetically coupled Os5+ and Mn2+ moments within the layers which then couple antiferromagnetically between layers. The ordered moments are 3.7(2) ýB for Mn and 1.9(2) ýB for Os and are parallel to the c-axis.

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