  The thermoelectric properties between 10 and 300 K and the growth of single
crystals of n-type and p-type GeBi4Te7, GeSb4Te7, and the Ge(Bi1-xSbx)4Te7
solid solution are reported. Single crystals were grown by the modified
Bridgman method, and p-type behavior was achieved by the substitution of Bi by
Sb in GeBi4Te7. The thermopower in the Ge(Bi1-xSbx)4Te7 solid solution ranges
from -117 muVK^-1 to +160 muVK^-1. The crossover from n-type to p-type is
continuous with increasing Sb content and is observed at x = 0.15. The highest
thermoelectric efficiencies among the tested n-type and p-type samples are ZnT
= 0.11 and ZpT = 0.20, respectively. For an optimal n-p couple in this alloy
system the composite figure of merit is ZnpT = 0.17 at room temperature.
