Study of Lattice Resonance Modes through Specific-Heat Measurements: KCl:Tl and KCl:Br

The specific heat of KCl doped with 1-, 3-, and 5-mole% TlCl and of KCl-KBr mixed crystals has been measured between 1 and 30 °K. The frequency associated with the resonance mode, caused by the heavy impurity, was determined to be 51 ± 8 ${\mathrm{cm}}^{-1\mathrm{}}$ (wave number) for KCl: Tl and 110 ± 10 ${\mathrm{cm}}^{-1\mathrm{}}$ for KCl: Br, in close agreement with the resonance frequencies determined from thermal conductivity and infrared absorption. Our findings support the picture that the impurity modes in these cases are determined to a good approximation by the different mass of the impurity and not by a change in the local binding forces.