Thermoelectric properties of (Ca, Sr, Bi)2Co2O5 whiskers : Special Issue on Structure and Electronic Properties in the Pseudogap Systems

Thermoelectric properties of (Ca, Sr, Bi) 2 Co 2 O 5 (Co-225) single crystalline whiskers with a layered structure were measured over a wide temperature range 100-973 K. Both Seebeck coefficient and electrical resistivity exhibited fairly complex temperature dependences in this temperature range. The whole temperature range studied is divided into four distinct regions (I) to (IV), depending on observed characteristic temperature dependences of both Seebeck coefficient and electrical resistivity. From more or less linearly temperature dependent Seebeck coefficient in region (I) in combination with unique temperature dependences of both resistivity and Hall coefficient, we conclude the presence of a small pseudogap with a width of a few meV across the Fermi level. Complex magnetic properties are observed: the antiferromagnetic transition at 22 K but the hysteresis in the M-H curve remains up to room temperature. This is taken as evidence for the existence of Co atoms situated in different magnetic environments. The possession of large Seebeck coefficients exceeding 100 μV K -1 in this system is attributed to the presence of the pseudogap at the Fermi level.