Anomalous behaviour of the low-temperature heat capacity for KCl crystals with heavy univalent impurities

The effect of low concentrations of heavy univalent Cs' and Iimpurities on the low-temperature heat capacity of KC1 gives an enhancement in the heat capacity. This anomalous behaviour has been explained on the basis of a low concentration Green's function theory. A localized perturbation model which includes mass change at the impurity site as well as changes in the nearest neighbour radial force constants has been employed in performing the calculations. Reasonable agreement with the experimental data is obtained. The mass defect contribution is found to be 80 % of the changed specific heat and the remaining contribution is due to force constant changes. The main contributions to the changed specific heat is caused by odd parity Flu-symmetry modes. The real part of the resonance denominator in Flu-irreducible representation gives a quasi localized frequency. The present value of the resonant frequency in case of KC1 : Iis seen to be similar to those obtained earlier in the study of thermal conductivity of the same system.