The thermal resistivity, Seebeck coefficient, electrical resistivity, and Hall mobility of Ge-Si alloys have been measured throughout the Ge-Si alloy system as functions of impurity concentration in the range 2×1018−4×1020cm−3, and of temperature in the range 300°–1300°K. A qualitative interpretation of these properties is given. For power conversion, boron and phosphorus were found to be useful p-type and n-type impurities, respectively, because of their high solid solubilities. At 1200°K, the maximum values of the dimensionless figure of merit zT were 0.8 for p-type Ge0.15-Si0.85 alloy doped to 2.1×1020cm−3 holes, and 1.0 for n-type Ge0.15-Si0.85 alloy doped to 2.7×1020cm−3 electrons. The maximum over-all efficiency of a stable generator operating between 300°–1200°K, using the best p-type and n-type materials was computed to be 10%.

CENTERS IN KBr CRYSTALS 767 bility of consecutive jumps such as (0,0,1) to (0,1,0) followed by (0,1,0) to (0, 0, -1) since this is reorientation by first going into the (100) configuration.The presence of trimers made up of three divalent metal vacancies was previously suggested by Cook and Dryden' to explain the decrease in dielectric loss on annealing crystals doped with divalent metal impurities.The trimer formation they suggested was a planar hexagon in a single (111) plane, which has no dipole moment, and would be almost impossible to reorient into another (111) plane. It should be noted that our "J.S. Cook and J. S. Dryden, Proc. Phys. Soc. (London) 80, 479 (1962). models for D1 and D2 both have dipole moments, and since they reorient fairly easily, it might be dificult to distinguish them from isolated impurity-vacancy pairs by dielectric loss measurements alone. ACKNOWLEDGMENTS We wish to thank R. Boulet for growing crystals, J. R. Morton and M. I'ujimoto for electron-spinresonance measurements, E. Whalley for suggesting the design of the high-pressure container, and J. H. Simpson for helpful discussion throughout the course of the experiments.

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