STUDY OF TUNNELING IMPURITIES IN ALKALI HALIDES AT LOW TEMPERATURES (THERMAL EXPANSION, LITHIUM DOPED POTASSIUM CHLORIDE, SPECIFIC HEAT, CYANIDE BROMIDE, DILATOMETRY)

The impurity tunneling systems of Li('+) in KCl and CN('-) in KBr have been studied by means of specific heat (C) and thermal expansion ((alpha)) measurements obtained at temperatures T between 0.09 K and 10 K. For the Li('+) defects, a peak occurs in the thermal expansion near 1 K which is qualitatively similar to the Schottky anomaly observed in the specific heat. For T > 0.6 K, the Gruneisen parameter, (GAMMA), (PROPORTIONAL) 3(alpha)/C, is isotropic and equal to +150 (+OR-) 15 independent of temperature and lithium isotope. At lower temperatures, (GAMMA) becomes temperature dependent and anisotropic with respect to crystal orientation, probably because of interactions between Li('+) sites. All measurements were for Li('+) concentrations < 200 ppm. The KBr:CN system has been studied for CN('-) concentrations ranging from 0.034% to 50%. At the lowest concentration, the thermal expansion consists of a positive peak near 1 K ((GAMMA) (DBLTURN) +50) and a negative peak near 0.2 K ((GAMMA) (DBLTURN) -100). As the CN('-) concentration is increased, the thermal expansion becomes smaller in magnitude and in temperature dependence as the orientational glass phase develops. At 50% concentration, the thermal expansion (with (GAMMA) (DBLTURN) +1) is similar to that of many amorphous solids.