Speculation on the Centres formed by Nitrogen in Diamond

Imperfections in type I and type II diamonds

Natural diamonds have been thinned sufficiently by oxidation at 750 °C in oxygen or at 1350 °C in carbon dioxide for examination by transmission electron microscopy. Type I and type II diamonds as classified by infra-red and ultra-violet absorption measurements have been investigated and a difference in the nature of the imperfections in the two types has been found. Impurity platelets are present on {100} planes in type I diamonds and the relevant impurity has been identified as most probably nitrogen, since the density of platelets can be explained only by the high concentrations of nitrogen found in such diamonds by other workers. The presence of the nitrogen in platelet form can explain anomalous X-ray spikes reported around the normal Laue spots and also the fact that the nitrogen has been found to be present in a non-paramagnetic form. Small dislocation loops have been revealed on {111} planes near the nitrogen platelets and have been attributed to the condensation of vacancies following the formation of the platelets. Both these types of imperfection are absent in type II diamonds. Defects common to both types are dislocations and also long narrow dislocation dipoles which are considered to be formed by the movement of screw dislocations containing long jogs through the crystal at high temperatures. It is suggested that type II diamonds may have grown in nitrogen-free conditions at temperatures similar to those required for type I diamonds and have cooled slowly or they may have grown at lower temperatures than type I diamonds.