Dresden 2006 – scientific programme

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MM: Metall- und Materialphysik

MM 19: Electronic Properties I

MM 19.5: Talk

Tuesday, March 28, 2006, 17:30–17:45, IFW B

Structural and Electronic properties of the Layered Na_0.46CoO₂ — •Dimitri Argyriou¹, L.C. Chapon², C. Milne¹, O. Prokhnenko¹, and P.G. Radaelli² — ¹Hahn-Meitner-Institut, Glienicker Str. 100, Berlin D-14109, Germany — ²ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, UK

The onset of antiferromagnetic ordering coupled to a sharp rise in the resistivity of Na_0.5CoO₂ has recently been taken as evidence of charge ordering in this half doped cobaltate[1]. We have used neutron powder diffraction to investigate the lattice response to this charge and magnetic ordering using high resolution powder diffraction from 2 till 600K. Our sample is a little off the ideal composition x=0.46, but shows the identical transitions at T_co=52K and at 87K, as other single crystal sample of nominal x=0.5 composition. We find that the orthorhombic structure of Na_{0. 46}CoO₂ (Pnmm) is stable from 460K to 2K, even below the charge ordering transition at 50K. At room temperature we can clearly identify two Co sites which are distinguishable from their octahedral distortions. One site, labeled Co(1) is at the center of a CoO₆ octahedron that shows an octahedral distortion 5 times larger than the octahedron centered in the Co(2) site. The average bondlength for both Co sites is essentially the same, 1.890Å, suggesting the same bond-valence. With lowering temperatures below T_co, we find no anomalies in Co-O bondlengths or octahedral distortion parameters, suggesting that the sharp increase in the resistivity at 50K may arise from an effect other than charge ordering. [1] M.L. Foo, et al. Phys. Rev. Lett. 92, 247001 (2004).