There was strong evidence to support the theory that the vacancy cluster, a by-product of dislocation interaction, was the extended defect responsible for brown colour in natural type IIa diamonds. A characteristic of these open volume structures was the hybridised sp2 bonding of the 3-fold coordinated carbon atoms that line their cage-like surfaces. When present in sufficiently large concentrations the sp2 bonds alter the electronic and optical properties of type IIa diamond dramatically. Similar sp2 bond concentrations were found in dark brown CVD diamond, although the defects in this material appear much larger in size than in the natural diamond. Core-loss electron energy loss spectroscopy and aberration corrected scanning transmission electron microscopy were used here to compare the relative sp2 content of brown coloured natural type IIa diamonds and dark brown CVD diamonds, using 100% sp2 bonded graphite as a reference. The effect of high-pressure high-temperature treatment on sp2 content was also investigated.

A Study of the Relationship between Brown Colour and Extended Defects in Diamond using Core-Loss Electron Energy Loss Spectroscopy. I.S.Godfrey, U.Bangert: Journal of Physics: Conference Series, 2012, 371[1], 012016