Structures and thermoelectric properties of the infinitely adaptive series (Bi2) m (Bi2 Te3) n

The structures and thermoelectric properties of the (Bi2) m (Bi2 Te3) n homologous series, derived from stacking hexagonal Bi2 and Bi2 Te3 blocks, are reported. The end members of this series are metallic Bi and semiconducting Bi2 Te3; 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 Bi2 Te3 to p type for phases rich in Bi2 blocks but with some Bi2 Te3 blocks present, to n type again for Bi metal. The thermoelectric power factor (S2 ρ) is highest for Bi metal (43 μW K2 cm at 130 K), followed by Bi2 Te3 (20 μW K2 cm at 270 K), while Bi2 Te (m:n=5:2) and Bi7 Te3 (m:n=15:6) have 9 μW K2 cm (at 240 K) and 11 μW K2 cm (at 270 K), respectively. The results of doping studies with Sb and Se into Bi2 Te are reported.