Dislocations, vacancies and the brown colour of CVD and natural diamond

The brown colouration of both natural and CVD diamond has attracted attention in recent years due to the ease by which it can be annealed out and thereby making the diamond more valuable. The earliest investigations linked the colouration to dislocations containing broken bonds at their cores since all natural brown diamonds show evidence of plastic deformation. The effect of the heat treatment was then thought to cause a change to their core structure by healing the bonds and eliminating their optical activity. However, this model suffers from several problems: the dislocation density in type IIa and especially CVD brown diamonds seems too low to explain the magnitude of the absorption, the most stable dislocations do not have broken bonds in their cores, and recent EELS studies show that the brown centres lie between slip bands. Another possibility is that the brown centres are multivacancy defects and there is support for this model from theoretical investigations, positron annihilation data, TEM and the observation of an enhanced growth of vacancy nitrogen defects in type Ia brown diamonds following high, > 2000 °C, treatments which lead to colour removal. It is considered that the vacancies are introduced into the diamond through climb of the dislocations explaining the link between the two. It is natural to suggest that the same centres are responsible for the brown colouration in CVD diamond: the absorption spectra of the two are very similar although the annealing conditions are quite different. However, recent experiments cast doubt on this suggestion.