Theoretical stability limit of diamond at ultrahigh pressure

Based on density-functional total-energy pseudopotential calculations, we find that diamond structure carbon becomes energetically unstable relative to a highly distorted tetrahedrally coordinated structure at pressures above 5.0 Mbar. This distorted phase has a rhombohedral crystal structure containing eight atoms (denoted as the R8 structure) and has recently been identified experimentally as a pressure-induced metastable phase of silicon. This result has profound implications for ultrahigh pressure experimentation as it places a substantially lower limit on the maximum pressures attainable with diamond-anvil cell technology than was believed to exist previously.