Electronic structures of hole-doped transition metal cyanides, Na 0:84Àx Co[Fe(CN) 6 ] 0:71 . 3.8H 2 O (NCF71), Na 0:72Àx Ni[Fe(CN) 6 ] 0:68 . 5.1H 2 O (NNF68) and Na 1:60Àx Co[Fe(CN) 6 ] 0:90 . 2.9H 2 O (NCF90),were investigated by means of the x-ray absorption spectroscopy and the valence differential spectroscopy. The x-ray absorption spectroscopy revealed that the holes are introduced on the Fe, Fe, and Co sites for the NCF71, NNF68 and NCF90 films, respectively. Owning to the valence differential spectroscopy, we unambiguously assigned the spectral components to the respective optical transitions. We further found that an ab initio band calculation based on the local density approximation with the onsite Columbic repulsion (LDA+U) semi-quantitatively explains the optical transitions.
A full-potential augmented-plane-wave (FLAPW) band-structure calculation in the local density approximation (LDA) was carried out for hexagonal Na(x)CoO(2) (x = 0.45, 0.55, 0.66 and 0.75). The Seebeck tensor was estimated by the Boltzmann theory, assuming that the relaxation time is constant on the Fermi surface. The Seebeck tensor is extremely anisotropic; the c-axis Seebeck coefficient varies dramatically with the Na content. The calculation reproduces the experiment semiquantitatively.
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