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Formation of anti-site defects and bismuth overstoichiometry in p-type Sb_2-xBi_xTe₃ thermoelectric crystals

Abstract
P-type Sb_2-xBixTe₃ crystals with various chemical compositions (x = 0.40, 0.44, 0.47, 0.50, 0.53, 0.56 and 0.60) were fabricated by zone melting method. Transmission electron microscopy and energy dispersive X-ray microanalysis were performed to characterize the evolutions of defects on the sites of Bi, Te and Sb atoms in the lattice. Study of Sb_2-xBi_xTe₃ structure reveals the formation of Bi₂Te₃ in which native defects are a consequence of overstoichiometry of Bi atoms in Bi₂Te₃ single crystal. Thermoelectric properties, including Seebeck coefficient (), electrical conductivity (), thermal conductivity (k), and Hall constant were measured at room temperature. In terms of thermoelectric properties, increasing Bi₂Te₃ content (x) decreased carrier (hole) concentration (and , as a result) and increased . The maximum figure-of-merit of 2.710^-3 K^-1 was obtained at about 300 K for 25%Bi₂Te₃ - 75%Sb₂Te₃ with 3 wt% excess Te recommended for thermoelectric properties modification. Added Te is considered not only as a dopant, but also to compensate the deficiency of Tellurium understoichiometry. The results compared with reports of several authors to evaluate the dependency of parameters on sites of the atoms. The novelty of this work is to present the sites of the atoms in the lattice of the ternary compounds and the effect of defects occurred due to the atomic sizes of the Bi, Sb and Te.