Phonon scattering by impurity platelet precipitates in diamond

An expression is obtained for the relaxation time for scattering of phonons by platelets of precipitated impurities of atomic masses and volume different from the host. For large-diameter platelets a closed expression was used in a model describing plateletlike nitrogen precipitates in type-$\mathrm{I}a$ diamonds. A fit of the model to thermal-conductivity data reported by Slack indicates that the major fraction of nitrogen impurities are present in nonparamagnetic point-defect associations rather than in large precipitates, at least for that specimen. The minor fraction of nitrogen present as platelets governs the thermal conductivity at very low temperatures.