Thermoelectric properties of (Ca, Sr, Bi)(2)Co2O5 whiskers

Thermoelectric properties of (Ca, Sr, Bi)(2)Co2O5 (Co-225) single crystalli ne whiskers with a layered structure were measured over a wide temperature range 100-973 K. Both Seebeck coefficient and electrical resistivity exhibi ted fairly complex temperature dependences in this temperature range. The w hole 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 wit h unique temperature dependences of both resistivity and Hall coefficient, we conclude the presence of a small pseudogap with a width of a few meV acr oss the Fermi level. Complex magnetic properties are observed: the antiferr omagnetic 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 ato ms situated in different magnetic environments. The possession of large See beck coefficients exceeding 100 muV K-1 in this system is attributed to the presence of the pseudogap at the Fermi level.