Positron lifetime results were obtained for brown and colourless natural diamond. Optical effects of the observed vacancy defects in brown high-pressure high-temperature treated colourless, and in naturally colourless type-IIa diamond, samples were studied by combining the positron measurement with monochromatic illumination. Brown diamond was found to contain optically active vacancy clusters (40 to 60 missing atoms); strongly correlated with the optical absorption spectra. The optical activity of these vacancy clusters was manifested by a photo-excitation induced change of charge from neutral to negative. The clusters gradually disappeared during high-pressure high-temperature treatments, and the samples treated at 2500C resembled colourless samples optically, and had similar positron lifetimes. The results showed that the brown colour originated from the vacancy clusters and that their removal by the high-pressure high-temperature treatment caused the loss of coloration.

Properties of Optically Active Vacancy Clusters in Type IIa Diamond. J.M.Mäki, F.Tuomisto, C.J.Kelly, D.Fisher, P.M.Martineau: Journal of Physics - Condensed Matter, 2009, 21[36], 364216