The structures and thermoelectric properties of the ${\left({\mathrm{Bi}}_2\right)}_m{\left({\mathrm{Bi}}_2{\mathrm{Te}}_3\right)}_n$ homologous series, derived from stacking hexagonal ${\mathrm{Bi}}_2$ and ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ blocks, are reported. The end members of this series are metallic Bi and semiconducting ${\mathrm{Bi}}_2{\mathrm{Te}}_3$; 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 $\left(S\right)$ that vary from $n$ type for ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ to $p$ type for phases rich in ${\mathrm{Bi}}_2$ blocks but with some ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ blocks present, to $n$ type again for Bi metal. The thermoelectric power factor $(S^2∕\rho )$ is highest for Bi metal ($43\mu \mathrm{W}∕{\mathrm{K}}^2\mathrm{cm}$ at $130\mathrm{K}$), followed by ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ ($20\mu \mathrm{W}∕{\mathrm{K}}^2\mathrm{cm}$ at $270\mathrm{K}$), while ${\mathrm{Bi}}_2\mathrm{Te}$ $(m:n=5:2)$ and ${\mathrm{Bi}}_7{\mathrm{Te}}_3$ $(m:n=15:6)$ have $9\mu \mathrm{W}∕{\mathrm{K}}^2\mathrm{cm}$ (at $240\mathrm{K}$) and $11\mu \mathrm{W}∕{\mathrm{K}}^2\mathrm{cm}$ (at $270\mathrm{K}$), respectively. The results of doping studies with Sb and Se into ${\mathrm{Bi}}_2\mathrm{Te}$ are reported.

• Received 17 December 2006

DOI:https://doi.org/10.1103/PhysRevB.75.195203