Thermodynamic properties of Coulomb crystals with isotopic impurities

This work is devoted to a description of Coulomb crystals with isotopic impurities which are expected to be formed in degenerate stars interiors. We consider the case when the mass of impurities M ' is slightly different from the mass of 'basic' ions M while fraction of impurities can be arbitrary. It is shown that the heat capacity perturbation theory of disordered crystals is in a good agreement with the linear mixing theory and with the exact calculations for binary crystals.