Diamond properties as a function of isotopic composition

Large single-crystal diamonds of varying isotopic composition were synthesized by combining chemical vapor deposition and high pressure technology. These crystals have an extremely high crystal quality, a low defect density and low internal stresses. Their thermal conductivity increases with increasing isotope purity with the maximum conductivities occurring with either isotopically pure 12C or 13C. The minimum thermal conductivity occurs symmetrically as a function of isotopic composition at 50% 13C. As the 13C concentration is increased, the interatomic spacing decreases, the two- and three-phonon absorption bands move towards lower wave numbers, and the frequency of the first-order Raman line decreases.