In situ X-ray diffraction study of the phase transition from graphite to hexagonal diamond under high pressures and high temperatures

Abstract

High pressure and high temperature in situ x‐ray diffraction experiments were carried out to study the phase transition from graphite to hexagonal diamond and the quenchability of hexagonal diamond to ambient conditions. When well crystallized graphite was compressed under quasi‐hydrostatic conditions, the phase transition to hexagonal diamond started at 18 GPa at room temperature, and this transition accelerated rapidly with increasing temperature. The hexagonal diamond formed at room temperature converted to graphite during the release of pressure, but when the sample was heated to more than 800 °C under high pressure, the hexagonal diamond phase became quenchable to ambient conditions. The observed 002 d‐spacing of the quenched hexagonal diamond is slightly larger than 111 d‐spacing of cubic diamond. Therefore, the c‐axis of hexagonal diamond is slightly expanded from an ideal structure based on the bond length in cubic diamond.