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The low-temperature heat capacity of cryocrystals containing impurity clusters is investigated theoretically and experimentally. Such defects might essentially enrich the low-frequency part of the phonon spectrum by introducing both localized and delocalized vibrations. The effect of both types of vibrations on the temperature dependence of the heat capacity is analyzed. The heat capacity of the disordered solid solution Kr–Ar (Ar concentration∼ 25%⁠) is studied as an example of the effect of the light, weakly coupled impurities on the low-temperature thermodynamic characteristics of a system. The mass defect of such an impurity induces “phonon pumping” from the low-frequency part of the spectrum into the high-frequency part and decreases the low-temperature heat capacity, while the weakened interaction between the impurity and the host atoms, combined with even weaker interaction between the impurities …