High-purity type-IIa synthetic crystals, containing less than 0.1ppm of impurities, were grown by using a temperature gradient method at high pressures and high temperatures. Crystal defects and internal strains were studied by means of double-crystal X-ray rocking curve measurements, polarizing microscopy, X-ray topography and Raman spectroscopy. The specimens were found to be of high crystalline quality, with fewer crystal defects, less internal strain and smaller crystal-to-crystal variations in defect content than in natural diamonds or synthetic type-Ib diamonds. However, some line and plane defects were observed in synthetic type-IIa crystals. It was found that, by using strain-free and low defect density crystals as seeds, line defects (dislocation bundles) could be avoided.

H.Sumiya, N.Toda, Y.Nishibayashi, S.Satoh: Journal of Crystal Growth, 1997, 178, 485-94