Solid carbon at high pressure: Electrical resistivity and phase transition

The electrical resistivity of polycrystalline graphite and amorphous carbon are measured at high pressures and room temperature. The results show that the resistivity of these carbon phases decreases with increasing pressure below 19 GPa. The pressure dependence of the resistivity (dln?/dP) is determined to be-0.037 GPa^?1 for the polycrystalline graphite and-0.039 GPa^?1 for the amorphous carbon. A phase transition, interpreted as the transformation to hexagonal diamond phase, is observed in the polycrystalline graphite at about 20 GPa but not in the amorphous carbon. The present experimental results support the assumption that this phase transition is martensitic in nature.