An unusual anisotropy of thermopower and magnetothermopower has been observed in the powerful thermoelectric Ca3Co4O9+δ single crystal. The in-plane thermopower is about twice as big as the out-of-plane thermopower. Combining ab initio band structure calculation with semiclassical model analysis, we understand this anisotropy with band structure effects and especially with anisotropic Fermi surface. We find that a strong anisotropy in the topology of Fermi surface leads to the anisotropy of (magneto)thermopower. This study may also shed light on anisotropic properties of other layered cobalt oxides.

Polycrystalline Ca 3-x Eu x Co 4-y Fe y O 9?d samples were synthesized by a solid-state reaction method and consolidated by spark plasma sintering. Effects of Eu/Fe dual doping on thermoelectric properties have been systematically investigated. The results indicate that the alteration of resistivity results from the change of carrier concentration, while the variety of thermopower originates from the spin entropy. The thermal transport process is mostly dominated by the point-defect scattering in this system. The transport properties imply that Ca 3-x Eu x Co 4-y Fe y O 9?d undergo a transition from the thermally activated semiconductor into two-dimensional variable range hopping semiconductor at x = 0.05 and y = 0.1. A larger ZT value of 0.027 at 300 K was achieved for Ca 2.95 Eu 0.05 Co 3.9 Fe 0.1 O 9?d , which is about 60 % larger than that of the pure Ca 3 Co 4 O 9?d . This investigation suggests that combining a co-doping approach and SPS technology is an effective path to enhance the ZT value for polycrystalline Ca 3 Co 4 O 9?d .