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The article discusses the result of annealed diamonds through low-pressure/high-temperature (LPHT) without the application of stabilizing pressure. The LPHT annealing was run in hydrogen plasma and was examined on chemical vapor deposition (CVD) grown single crystals in temperatures ranging from 1,400 to 2,200 degree Centigrade. In addition, the LPHT annealing controls high-temperature treatment of diamond surfaces which is important for the micro- and nanostructuring of diamonds.

Low-pressure, high-temperature (LPHT) annealing of yellow-to-brown type Ia natural diamonds was performed to monitor its effects on optical centers within diamond, changes in the observed color, and to assess the process's viability as a commercial gem treatment. With LPHT annealing only, the mostly brown diamonds showed a shift towards yellow coloration; Vis-NIR absorption spectra showed this change was due to a modest increase in H3 intensity. Even at long annealing times (24 h at 1800 °C) or annealing at high temperatures (2000 °C for five minutes), the diamonds did not significantly lose brown coloration. LPHT annealing showed itself as an ineffective means to break apart the vacancy clusters causing the brown color or causing nitrogen disaggregation, which resulted in only a small H3 generation. With LPHT annealing, “amber centers”—a group of several independent bands in the IR between 4200 and 4000 cm − 1 that disappear with HPHT annealing—were seen to anneal out gradually at various temperatures from 1700 to 2000 °C. In contrast, high-pressure, high-temperature (HPHT) annealing effectively removes brown color at similar time/temperature conditions. Without the high stabilizing pressure provided by HPHT annealing techniques, the LPHT annealing showed pronounced damage on inclusions and dramatic surface etching. In subsequent experiments, LPHT annealing was used as a follow-up to laboratory irradiation. The irradiation-related vacancies created greater concentrations of H3 and the vacancy-assisted disaggregation of nitrogen created donors which led to a high concentration of H2 centers. This combination of defects resulted in a pronounced and favorable shift towards saleable yellow colors due to an increase in H3 and a dramatic increase in the H2 center, which led to the suppression of the remaining brownish component. The annealing characteristics for many centers detected by Vis-NIR absorption spectroscopy, FTIR absorption spectroscopy, and photoluminescence spectroscopy were chronicled throughout the study and compared with other LPHT annealing studies and HPHT annealing experiments. [ABSTRACT FROM AUTHOR]

This study gives an analysis of the effect of low-pressure, high-temperature annealing on the infrared, Raman, and photoluminescence (PL) features, as well as the inclusion characteristics, of cubo-octahedral diamond plates from the Marange deposits in Zimbabwe. The samples showed strong inclusion-related zoning which consists of micron-sized particles identified as graphite and these grew noticeably larger with annealing at temperatures of 300 °C to 1700 °C. Within the natural diamonds, the graphite inclusions (detected by Raman spectroscopy) had a grain size of approximately 1 μm, which increased to 3 μm after 1200 °C and 14 μm after 1700 °C annealing and their hexagonal morphology was discernible. From the geometry of these grains, we determined that they were oriented within the {111} family of planes. The infrared absorption and PL spatial maps were collected after every temperature step to study the effects of annealing on the defects, and photomicrographs and Raman spectra were collected to study the graphite inclusions. The graphitic inclusions grew much larger as the stressed diamond surrounding them converted to graphite. Many nitrogen-related optical centers, including NV − and H3 are no longer detected after high temperature annealing within the cuboid regions as these may have been transformed to hydrogen-bearing complexes such as NVH and N 2 VH. The presence of CH 4 is detected in the unannealed Marange diamonds, but was no longer observed in Raman spectra after 1200 °C annealing. This CH 4 disappearance along with changes in inclusion morphology could provide a method to detect heat treatment if these mixed-habit samples are sourced to create treated black gem diamond. [ABSTRACT FROM AUTHOR]