High-pressure phases of calcium: Prediction of phase VI and upper-pressure phases from first principles

Comparing enthalpies among the crystal structures which have recently been determined by experiments or been predicted by first-principles calculations, we have reconstructed the phase diagram of calcium and found further high-pressure phases. Our phase diagram predicts that the phase V of an orthorhombic Cmca structure exists only in a small pressure range narrower than 10 GPa and it transforms to another phase, phase VI of an orthorhombic Pnma structure. In addition, we found that the phase VI prevails about 20 GPa range and transforms to still another phase, phase VII of a tetragonal I4/mcm(00 gamma) structure at 135 GPa and finally takes the hexagonal-close-packed structure above 495 GPa. Calculated results of the superconducting transition temperature T-c on the basis of our phase diagram suggest that T-c of 25 K, which is the experimentally reported highest value in calcium, should have been observed in the phase VI or phase VII, rather than in phase V.