Electronic Properties of 2D Alkali Cobaltites and Related Oxides

Strong thermoelectric power (TEP) can be achieved in semiconducting oxides when a low carrier density is created by appropriate atomic substitutions or intercalations, but the electrical resistivity remains too large. Actually, oxides such as layered cobaltites for which the best balance between high TEP and small resistivity is reached contain a large concentration of strongly correlated carriers. However, the origin of the large TEP values still remains an open question. In mixed valence oxides two main transport mechanisms are generally involved: either a metallic type transport with a mean free path larger than that predicted by the Ioffe-Regel limit or a hopping type transport. In the first case, according to Mott's equation, large TEP values could result from peculiar energy dependence of the density of states and relaxation time at the Fermi level; in the second case TEP can be calculated using Heikes formula. In both cases, TEP can be enhanced by spin entropy effects mainly expected for spin polarized metallic oxides (or half metals) rather than Pauli metals. For hopping transport an additional term in Heikes formula arising from the spin as well as orbital degeneracy must be taken into account. The specificity of the Co
³⁺
ions (3d
⁶
) that can exhibit three different electronic configurations (S = 0, S = 1 and S = 2) in oxides, depending on the interplay of exchange and crystal field energies, as well as the dimensionality of the crystal structure, the site symmetry and correlation effects is also discussed. The behavior of recently investigated potassium-intercalated 2D-cobaltites is compared to that of the corresponding sodium oxides