Effect of Sm-doping on microstructure and thermoelectric properties of textured n-type Bi2Te2.7Se0.3 compound due to change in ionic bonding fraction

Textured n-type Bi2-xSmxTe2.7Se0.3 compounds with x ​= ​0; 0.005; 0.02 and 0.05 have been prepared by microwave-solvothermal synthesis of starting powders and spark plasma sintering of bulk materials. Texturing [001] axis coincided with direction of spark plasma sintering pressuring. With increasing x, (i) an average lateral size of grains in the samples is decreasing; (ii) the Lotgering factor characterizing a degree of preferential orientation of the grains is increasing; (iii) anomalies of all the thermoelectric properties, related to onset of intrinsic conductivity, are shifted to higher temperatures. The Sm-doping effect on the grain structure and thermoelectric properties of samples is attributed to increasing in ionic bonding fraction, which can occur at increasing Sm content due to a difference in electronegativity of Bi and Sm atoms. Maxima in temperature dependences of the thermoelectric figure-of-merit, typical for Bi2Te3-based compounds and due to the intrinsic conductivity, were observed. The highest value of the thermoelectric figure-of-merit was found to be weakly dependent on x and falling into ~0.6 ÷ ~0.7 interval, whereas temperature position of the maxima is remarkably shifted to higher temperatures with increasing Sm content.