An experimental and theoretical investigation was made of low-loss electron energy losses in brown type-IIa monocrystalline diamonds, before and after high-temperature high-pressure annealing which removed the brown coloration. Additional losses were found, within and near, the band-edge for brown diamond which were significantly reduced after treatment. The additional losses were not associated with dislocations. Graphitic inclusions were detected by electron energy-loss spectroscopy as well as transmission electron microscopic studies for some brown diamonds before treatment. These led to pronounced sub-gap absorption. However, all brown diamonds exhibited additional losses which were due to point defects lying in the regions between dislocations. First principles theoretical modelling showed that common dislocations were not responsible for the brown coloration but a π-bonded vacancy disk lying on {111} planes gives broad bands lying in the diamond band gap, possesses an optical absorption spectrum similar to that of brown diamond, and led to additional electron energy losses in the band edge region. These and similar defects were suggested to be responsible for the brown coloration. Mechanisms were proposed for their formation and removal.

Electron Energy Loss Spectroscopic Studies of Brown Diamonds. U.Bangert, R.Barnes, L.S.Hounsome, R.Jones, A.T.Blumenau, P.R.Briddon, M.J.Shaw, S.Öberg: Philosophical Magazine, 2006, 86[29-31], 4757-79