Spatial distribution of defects in natural type IIb diamond after irradiation and annealing

Natural blue diamonds are among the rarest and most valuable of gemstones; however, they are occasionally treated by a variety of methods to improve the color or in attempts to obscure the evidence of treatment. In this study, two sections were cut from a rough naturally sourced type IIb diamond. One was subjected to electron irradiation along one edge. Subsequently, both were isochronally annealed from 300 °C to 1200 °C and the optical defects were documented by changes in infrared (IR) absorption and photoluminescence spectroscopic mapping. The thermal behavior of these centers (TR12, 3H, NV0, GR1, 648.2 and 776.4 nm centers among others) along with their spatial distribution in the irradiated and non-irradiated diamond provided additional insights into their configuration. After irradiation, the uncompensated boron concentration as determined by IR spectroscopy for this sample showed a pronounced decrease particularly at positions close to the irradiation edge. This uncompensated boron decreased further with low temperature annealing (300–600 °C) principally due to interstitial migration. At temperatures >600 °C, the uncompensated boron concentration rebounded due to vacancy migration that likely depletes or annihilates compensating defects. Among the photoluminescence (PL) defects, the 3H center (ZPL = 503.5 nm) showed a much greater thermal stability in the irradiated sample compared to its non-irradiated counterpart.