Light-Transparent Phase Formed by Room-Temperature Compression of Graphite

Single-crystal graphite has been compressed at room temperature and found to undergo a transformation at 18 gigapascals as indicated by a drastic increase in the optical transmittance. This high-pressure phase is unquenchable; a more transparent phase formed by the heating of this phase can be quenched at ambient conditions. This latter phase is probably "hexagonal diamond," and the present observations suggest that the structure of the transparent phase obtained by the room-temperature compression is different from the hexagonal diamond structure.