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On the origin of the metallic and anisotropic magnetic properties of NaxCoO2 (x≈0.75)

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Whangbo,  M. H.
Former Departments, Max Planck Institute for Solid State Research, Max Planck Society;
Department Nanochemistry (Bettina V. Lotsch), Max Planck Institute for Solid State Research, Max Planck Society;

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Kremer,  R. K.
Former Scientific Facilities, Max Planck Institute for Solid State Research, Max Planck Society;

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Citation

Whangbo, M. H., Dai, D., & Kremer, R. K. (2006). On the origin of the metallic and anisotropic magnetic properties of NaxCoO2 (x≈0.75). Inorganic Chemistry, 45(15), 5989-5995.


Cite as: https://hdl.handle.net/21.11116/0000-000E-FB8C-A
Abstract
Nonstoichiometric NaxCoO2 (0.5 < x < 1) consists of CoO2 layers made up
of edge-sharing CoO6 octahedra and exhibits strongly anisotropic
magnetic susceptibilities as well as metallic properties. A modified
Curie-Weiss law was proposed for systems containing anisotropic
magnetic ions to analyze the magnetic susceptibilities of NaxCoO2 (x
approximate to 0.75), and implications of this analysis were explored.
Our study shows that the low-spin Co4+ (S = 1/2) ions of NaxCoO2
generated by the Na vacancies cause the anisotropic magnetic properties
of NaxCoO2 and suggests that the six nearest-neighbor Co3+ ions of each
Co4+ ion adopt the intermediate-spin electron configuration, thereby
behaving magnetically like low-spin Co4+ ions. The Weiss temperature of
NaxCoO2 is more negative along the direction of the lower g factor
(i.e., theta(parallel to) < theta(perpendicular to) < 0 and g(parallel
to) < g(perpendicular to)). The occurrence of intermediate-spin Co3+
ions surrounding each Co4+ ion accounts for the apparently puzzling
magnetic properties of NaxCoO2 (x approximate to 0.75), i.e., the large
negative Weiss temperature, the three-dimensional antiferromagnetic
ordering below similar to 22 K, and the metallic properties. The
picture of the magnetic structure derived from neutron scattering
studies below similar to 22 K is in apparent conflict with that deduced
from magnetic susceptibility measurements between similar to 50 and 300
K. These conflicting pictures are resolved by noting that the spin
exchange between Co3+ ions is more strongly antiferromagnetic than that
between Co4+ and Co3+ ions.