Intrinsically modified thermoelectric performance of alkaline-earth isovalently substituted [Bi(2)AE(2)O(4)][CoO2](y) single crystals

Alkaline-earth elements isovalently substituted into a [Bi(2)AE(2)O(4)][CoO2](y) (AE(2) = Ca-2, Sr-2, and CaSr) single crystal with a layered structure were grown by the optical floating zone method. Structural characterization by X-ray diffraction and electron microscopy showed that the layers were oriented perpendicular to the c-axis, as well as the growth of direction was parallel to the ab-plane. The thermoelectric properties, including the Seebeck effect, electrical conductivity and thermal conductivity were investigated. The results of the thermoelectric measurements showed that the full substitution of Ca for Sr in [Bi2Sr2-xCaxO4][CoO2](y) has the best overall thermoelectric performance. Compared with the other two cases studied, the full Ca substituted crystal [Bi2Ca2O4][CoO2](y) exhibits both reduced resistivity and thermal conductivity, but not a reduced Seebeck coefficient. The enhanced thermoelectric property in [Bi2Ca2O4][CoO2](y) is mainly due to lower structural symmetry, which is confirmed by electron microscopy characterization. This work demonstrates that even isovalently substitution can play a crucial role in the thermoelectric effect of layered cobalt oxides. (C) 2013 AIP Publishing LLC.