Thermal-conductivity reduction in electron-irradiated type-IIadiamonds at low temperatures

The low-temperature thermal conductivity of three type-II $a$ diamonds irradiated with 0.75-MeV electrons at room temperature and one irradiated at 77 K was found to be reduced and is characteristic of phonon scattering by large aggregates: the conductivity changing from Rayleigh scattering at the lowest temperatures to geometrical scattering at higher temperatures. The size and concentration of these irradiation-produced aggregates were determined by fitting a theoretical model to the experimental results. Calculations based on the number of displaced atoms and on the number needed to form aggregates show that the formation of aggregates is possible but that the aggregates are limited to certain shapes and densities. The results indicate that the interstitial in diamond becomes mobile at a temperature between 77 K and room temperature in agreement with the recent suggestion of Lomer and Wild.