Winter 2010

Featured Lab Note: Silicon-Vacancy Defect Found in Blue HPHT-Grown Synthetic Diamond

Gem-quality type IIb synthetic diamonds, which contain traces of boron and usually have a blue color, can be created by both HPHT and CVD growth processes. The Si-related lattice defect is widely considered an identifying feature of CVD synthetic diamond. At the New York lab, however, we recently tested an HPHT-grown synthetic type IIb diamond that contained this defect.

The 0.30 ct round brilliant (4.43 × 4.33 × 2.67 mm) was color graded Vivid blue (top figure). Like other HPHT-grown synthetic diamonds, it contained some pinpoint inclusions and showed uneven color distribution along growth sectors that were clearly revealed in DiamondView fluorescence images (bottom figure). These were the only internal features seen. The mid-IR absorption spectrum showed a relatively high concentration of boron, consistent with the intense blue coloration. However, PL spectroscopy at liquid-nitrogen temperature using 514 nm laser excitation revealed clear Si-related emission lines. The sharp peaks at 736.7 and 737.0 nm, attributed to a [Si-V]^– lattice defect, closely matched those observed in CVD synthetic diamonds.

This is the first documented [Si-V]^– defect in an HPHT-grown synthetic gem diamond. Its occurrence strongly suggests some post-growth treatment process. During treatment, an existing Si impurity could have combined with other optical centers such as a vacancy and formed the [Si-V]^– defect. As always, care is required in identifying synthetic diamonds, particularly when post-growth treatment is involved.

Kyaw Soe Moe and Wuyi Wang