The reversible transition of graphite under high pressure: implications for the kinetic stability of lonsdaleite at intermediate temperature
Chien-Min Sung, Ming-Fong Tai
Most of the basal planes in graphite are in the 2H sequence, which cannot be converted directly to either lonsdaleite or diamond. The basal planes must be resequenced to 2H’ or 3R before graphite can transform into lonsdaleite or diamond, respectively. At ambient temperature, the thermal energy is insufficient for this resequence. Without the resequencing of basal planes, graphite is compressed to form a structure with 1/7 of its bonds in sp3 form, and the remainder in distorted sp2p form. The sp3 bonds make the high-pressure phase optically transparent and electrically insulating. The dominant sp2p bonds exhibit graphite-like Raman spectra. The majority distorted sp2p bonds destabilise the structure upon decompression. As a result, the minority sp3 bonds break, so the high-pressure phase reverts back to graphite at ambient pressure.