Formation of anti-site defects and bismuth overstoichiometry in p-type Sb2−xBixTe3thermoelectric crystals

Abstract

P -type Sb 2− x Bi x Te 3 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− x Bi x Te 3 structure reveals the formation of Bi 2 Te 3 in which native defects are a consequence of overstoichiometry of Bi atoms in Bi 2 Te 3 single crystal. Thermoelectric properties, including Seebeck coefficient (α), electrical conductivity (σ), thermal conductivity ( k ), and Hall constant were measured at room temperature. In terms of thermoelectric properties, increasing Bi 2 Te 3 content ( x ) decreased carrier (hole) concentration (and σ, as a result) and increased α. The maximum figure-of-merit ( Z = α 2 σ/ k ) of 2.7×10 −3 K −1 was obtained at about 300 K for 25%Bi 2 Te 3 − 75%Sb 2 Te 3 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.