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There is strong evidence to support the theory that the vacancy cluster, a by-product of dislocation interaction, is the extended defect responsible for brown colour in natural type IIa diamonds. A characteristic of these open volume structures is 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 are found in dark brown CVD diamond, although the defects in this material appear much larger in size than in the natural diamond. In this study we employ core-loss electron energy loss spectroscopy (EELS) and aberration corrected scanning transmission electron microscopy (AC-STEM) 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 (HPHT) treatment on sp2 content is also investigated.

A Study of the relationship between brown colour and extended defects in diamond using core-loss electron energy loss spectroscopy