Thermoelectric power of bulk (Bi,Pb)2-2:2:2:3 cuprates with the 2a×2b×c superstructure

Based on the powder x-ray and TEM analyses, the bulk 2a×2b×c superstructure of tetracyanoquinodimethane (TCNQ) treated (Bi,Pb)-2:2:2:3 cuprates has been attributed to an ordering of oxygen vacancies in the covalent CuO2 layers and is also likely in the ionic SrO layers. Both electrical resistivity and thermopower for the (Bi,Pb)-2:2:2:3 cuprate undergo dramatic changes after TCNQ treatment. Electrical conductivity of TCNQ-treated (Bi,Pb)-2:2:2:3 cuprates with bulk 2a×2b×c superstructure shows a metal-like temperature dependence with high resistivity (102-103 Ω cm), and a relatively rapid resistivity drop at ∼11.7 K. High resistivity of this material indicates a significantly reduced hole carrier concentration in the CuO2 layers. The thermopower is positive and exhibits a metallic-diffusion behavior, resembling the thermopower Sc in the c direction for Bi-2:2:1:2 single crystals. The disappearance of peak thermopower in this material suggests that the commonly observed peak behavior in cuprate superconductors is associated with the hole carriers in the CuO2 layers. In the framework of Kaiser's metallic-diffusion thermopower model, it seems that the abnormally large electron-phonon enhancement effect on the thermopower arising from the CuO2 layers in the pristine (Bi,Pb)-2:2:2:3 cuprate is significantly reduced in the TCNQ-treated samples with the 2a×2b×c superstructure. This could be due to a reduced in-plane conductivity contribution as a result of one-fourth of the oxygen missing in the CuO2 layers.