Abstract
Reversible phase transformations with volume contractions were found at 18 GPa and 22 GPa when graphite was compressed at room temperature. Volume contraction was detected in a diamond anvil cell by observing a plateau in the curve of generated pressure versus the cell load. The phase realized at 18 GPa is thought to work as an intermediate phase in the transformation from graphite to hexagonal diamond. A structure of this phase, though not yet identified, is also discussed.