Recent experiments have shown that vacancy clusters are common defects in all types of monocrystalline brown diamond. This is believed to be due to an enhanced stability of the bounding 〈111〉 or 〈110〉 surfaces, arising from the formation of π‐bonded chains. These chains lead to broad bands of gap states with acceptor and donor levels around Ev + 2 eV. They also are likely to be responsible for the broad absorption continuum extending from the NIR through the visible and into the UV region which is characteristic of brown diamonds. High temperature treatment removes or transforms the clusters in CVD and type IIa diamonds leaving the diamond colourless. In type Ia brown diamonds, heat treatments up to 2500 °C lead to the breakup of the clusters and the formation of specific vacancy‐nitrogen defects. We show that ab initio modelling of the interaction of nitrogen with the chains open a gap and leads to a threshold absorption energy in the 1–2 eV range. Finally, the calculations shed light on the identity of the vacancy‐nitrogen defects observed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The absorption spectrum of brown diamond is broad and featureless, in both natural type II and CVD-grown material. It is argued that such an absorption is due to an extended, rather than a point, defect. Ab initio modelling studies have been conducted on dislocations and extended vacancy-related defects. While certain dislocations could potentially explain the origin of colour, their density is too low to account for experimentally observed absorption magnitudes. It is demonstrated that a vacancy disk lying in the {111} plane has an absorption spectrum similar to that seen in natural and CVD brown diamond. Such disks are unstable above about 200 vacancies and should relax to dislocation loops in natural diamond. Hydrogen is shown to passivate the optical activity of the disks.

We investigate both experimentally and theoretically, low-loss electron energy losses in brown type IIa monocrystalline diamonds both before and after high-temperature, high-pressure anneals which remove the brown colouration. We find additional losses within and near the band edge for brown diamond which are significantly reduced after treatment. The additional losses are not associated with dislocations. Graphitic inclusions are detected by EELS as well as TEM studies for some brown diamonds before treatment. These lead to pronounced subgap absorption. However, all brown diamonds exhibit additional losses which are due to point defects lying in the regions between dislocations. First principles theoretical modelling shows that common dislocations are not responsible for the brown colouration but a pi-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 leads to additional electron energy losses in the band edge region. These and similar defects are suggested to be responsible for the brown colouration. Mechanisms are proposed for their formation and removal.

Measurements of the absorption spectra of brown natural type IIa diamond as well as brown nitrogen-doped CVD diamond are reported. These are largely featureless and increase almost monotonically from about 1-5.5 eV. It is argued that the brown coloration is due to an extended defect and not to a point defect. First principles modeling studies demonstrate that the spectra could be attributed to vacancy disks lying on {111} planes. Such disks are unstable above about 200 vacancies and should relax to dislocation loops in natural diamond. Hydrogen is shown to passivate the optical activity of the disks.

Ab Initio calculations, using density functional theory within the AIMPRO code, have been performed on bulk diamond and silicon in order to calculate the change in the dielectric constant due to strain, and hence the values of the photoelastic tensors. A combination of compressive and tensile strain has been applied to two atom unit cells of Si and diamond and the tensors relating to p(11) + 2p(12), p(12) - 2p(11) and p(44) extracted. Good agreement is found with experimental results for absolute values and frequency dependence of the constants for silicon. Results are also given for diamond where experimental data is much more limited and scattered. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

110 vacancy chains, multi-vacancy clusters and vacancy discs have been modeled using density functional theory within the AIMPRO and DFTB codes. While a connection can be established between the results on vacancy chains and previous EPR experiments, no connection can be made between the point defects and the optical properties of natural type IIa brown diamonds. However, a vacancy disc consisting of a {111} double plane of vacancies is stable and possesses an absorption spectrum similar to that found in brown diamonds. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.