No Jahn-Teller Distortion of the Lattice Structure Found in the Cobalt Oxide
La1-xSrxCoO3
summary written by Raven Hanna
The arrangement of atoms in molecules and complexes that include atoms with many interacting electrons can be hard to predict. The Jahn-Teller (JT) effect sometimes predicts a geometric distortion of the oxygen octahedra surrounding transition metals such as Mn, Co, and Cu. In this model, for certain ground state configurations of the electrons on the metal atom, the total electronic energy can be reduced if the surrounding oxygen octahedra adopt a distorted structure that might seem unstable. The JT distortion is seen in many copper complexes, produces a metal-insulator transition in many manganites, and was predicted to affect the shape of the oxygen octahedra about Co (CoO6) in La1-xSrxCoO3.
A research group led by Frank (Bud) Bridges from UC Santa Cruz used SSRL Beam Line 10-2 to test this prediction. The group is interested in the cobalt oxide La1-xSrxCoO3 (LSCO) because it has unusual magnetic properties that are not well understood. They used extended x-ray absorption fine structure (EXAFS) spectroscopy and neutron pair distribution function (PDF) techniques to determine variations in the Co-O bond lengths in powdered samples as a function of temperature. Their data can be fit very well to the classic correlated Debye model without invoking any JT distortions in contrast to the manganites which show a large JT distortion. They therefore conclude that significant JT distortions are not present in the cobalt oxide LSCO.
The lack of a significant JT distortion of the oxygen octahedral about Co provides evidence against a proposed intermediate spin state for Co. Consequently, the observed unusual magnetism must arise from a mixture of strongly magnetic, high spin states and non-magnetic low spin states. This work was published in the January 16 issue (2009) of the journal Physical Review Letters.
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"Local Structure of La1-xSrxCoO3 Determined from EXAFS and Neutron Pair Distribution Function Studies", N. Sundarum, Y. Jiang, I.E. Anderson, D.P. Belanger, C.H. Booth, F. Bridges, J.F. Mitchell, Th. Proffen, and H. Zheng, PRL 102, 026401, (2009)
