The True Catalyst for Diamond Formation: Tellurium or Carbon-Tellurium Compound?
The sintering behavior of polycrystalline diamond with a tellurium (Te) catalyst was studied at 10 GPa and 1750-1900 °C. CALYPSO software was used to find three crystal structures of carbon-tellurium (C-Te) compounds: CTe, CTe 2 , and C 3 Te 8 . The experimental results indicate that at 10 GPa, Te reacts with diamond only above 1850 °C. The C-Te compounds decompose into diamond and Te during cooling and pressure release. The Te from decomposition has a different crystal structure from those unreacted. Rapid cooling can retain some C-Te compounds at room temperature and pressure. By comparing the X-ray diffraction patterns of the C-Te compounds separated from the sample's spectrum with those of the three calculated crystal structures, it is suggested that the C-Te compounds in recovered sample are most likely orthorhombic CTe 2 . And computational results suggest that TeC may be the true catalyst in the diamond formation process. In samples sintered at 10 GPa, 1850 °C and 1900 °C, grain boundaries between diamond grains disappear. Hardness tests show these samples have an average Vickers hardness of about 61 GPa, similar to commercial polycrystalline diamond compacts.
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