Structures and thermoelectric properties of the infinitely adaptive series (Bi2)m(Bi2Te3)n

The structures and thermoelectric properties of the (Bi-2)(m)(Bi2Te3)(n) homologous series, derived from stacking hexagonal Bi-2 and Bi2Te3 blocks, are reported. The end members of this series are metallic Bi and semiconducting Bi2Te3; nine members of the series have been studied. The structures form an infinitely adaptive series and a unified structural description based on a modulated structure approach is presented. The as-synthesized samples have thermopowers (S) that vary from n type for Bi2Te3 to p type for phases rich in Bi-2 blocks but with some Bi2Te3 blocks present, to n type again for Bi metal. The thermoelectric power factor (S-2/rho) is highest for Bi metal (43 mu W/K-2 cm at 130 K), followed by Bi2Te3 (20 mu W/K-2 cm at 270 K), while Bi2Te (m:n=5:2) and Bi7Te3 (m:n=15:6) have 9 mu W/K-2 cm (at 240 K) and 11 mu W/K-2 cm (at 270 K), respectively. The results of doping studies with Sb and Se into Bi2Te are reported.