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For several years the ESRF, the University of the Witwatersrand and De Beers Industrial Diamonds (PTY) Ltd through their Diamond Research Laboratories have pursued a development programme to improve the quality of synthetic diamond crystals. Recently, in an effort to study the influence of nitrogen impurities on the defect structure, x-ray excited optical luminescence and spatially resolved double-crystal diffractometry were employed as new techniques. Finally, the impurity concentration distribution obtained by optical spectroscopy was superimposed on the defect structure determined by x-ray diffraction. The correlation between nitrogen impurities and the raw defect structure was clearly visible. It was confirmed that concentration variations are related to lattice imperfections, where tilts are much superior to lattice parameter variations.

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NMR measurements on single crystals of pure C diamond are reported. The line shape is very sensitive to the orientation of the external magnetic Beld relative to the crystallographic axes, typical of dipolar line broadening. With the magnetic field oriented along the [110] direction, the line is broad and fiat, whereas a more narrow, Gaussian shape is seen along [001]. For the [111] direction, a spectacular line splitting of 8.5 kHz is observed. For the sample studied, the spin-lattice relaxation time vras about 15 s at room temperature and 140 s at 140 K. The spectra are interpreted by a simple model using dipolar interactions.

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