Thermoelectric [Sr2(Bi,Co)n−2On]RS[CoO2]1.8 layer cobaltites: The role of the intergrowth between the n=3 and n=4 terms

The Bi–Sr–Co–O system has been investigated for small bismuth contents in order to study the formation of the misfit oxide series built up from the stacking of three rocksalt-type layers (n=3) with a single CdI2-type CoO2 hexagonal layer. Starting from nominal compositions, written as [Sr2Co1−xBixO3]RS[CoO2]1.8 (x=0.1, 0.2, 0.3, 0.4, and 0.5), samples have been synthesized and their structure has been analyzed by combining transmission electron microscopy techniques and powder x-ray diffraction data. For x≈0.5, intergrowths are clearly observed, made of a majority of triple (n=3) and a few quadruple (n=4) rocksalt-type layer-based misfit cobaltites. Transport measurements (2.5 up to 320 K) reveal that the Bi introduction reduces the resistivity, especially at room temperature. Furthermore, the thermopower (Seebeck coefficient S) of such intergrowth (S≈120μVK−1 at 300 K) is found to be of the same order as the one measured for the n=4 Bi–Sr cobaltite while the resistivity (ρ≈20mΩcm at 300 K) is significantly reduced in comparison with the same compound.