College Park, Maryland June 6 - 10 , 2004
T3-A5 (5:00 PM): Structure and Magnetism in Sr-Doped Rare Earth Cobaltates
M. James (Bragg Institute, Australian Nuclear Science and Technology Organisation, Australia), D. J. Goossens, R. L. Withers (Research School of Chemistry, Australian National University, Canberra, ACT 0200, AUSTRALIA)
Substantial interest has recently been generated by rare earth cobaltate compounds as cathode materials for solid oxide fuel cells. We have synthesized a wide range of single- phase perovskite-based rare earth cobaltates (Ln1-xSrxCoO3-d) (Ln = La3+, Pr3+, Nd3+, Sm3+, Gd3+, Dy3+, Y3+, Ho3+, Er3+, Tm3+ and Yb3+; 0.60 < x < 1.00). For the first time we have mapped the perovskite phase boundaries for these materials (synthesized under 1 atmosphere of oxygen). A combination of electron and x-ray diffraction of these compounds reveals three different classes of tetragonal superstructures. A phase boundary exists between compounds containing large and small rare earths (between Nd3+ and Sm3+) and also at the highest Sr-doping levels (x > 0.90). Powder neutron diffraction has been used in conjunction with the other diffraction techniques to reveal cation and oxygen vacancy ordering within these materials.
These phases show mixed valence (3+/4+) cobalt oxidation states that increases with Sr content; exceeding 50 % Co(IV) for the compositions Ln0.05Sr0.95CoO3-d. A range of magnetic behaviors has been observed using susceptibility techniques and in a number of instances we have observed ordered ferromagnetism at elevated temperatures (~ 300 K). In these cases, the magnetic structures of these materials were further characterized by powder neutron diffraction.
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