Lattice thermal conductivity in mixed crystals in the absence of mass mismatch: Investigation of N-2-CO solid solution

Mutual solid solution of nitrogen and carbon monoxide is a unique object in terms of phonon spectroscopy investigations. The main reason for phonon interaction with foreign admixture molecule-the difference of masses of the host and the impurity molecule-is absent from that system. In the present paper the results of measurements of thermal conductivity of pure and carbon monoxide-doped nitrogen crystals, for samples containing up to 50% of CO, in the temperature range 1.2-26 K, are presented. For analysis of the experimental data the Callaway approach was applied. It has been found that the dependence of relaxation time of phonon in its interaction with varying force constants and deformation of lattice around the admixture molecule on the phonon frequency agrees with earlier theoretical considerations, however the effect is weak. It has also been found that, for concentration of the admixture molecules exceeding 1%, the mutual interaction of the impurity molecules becomes significant and lowers the thermal resistivity per unit concentration of the admixture molecule.