Phases of Ca from first principles

Abstract

Structures and properties of many of the phases of Ca under pressure are calculatedfrom first principles by a systematic procedure that minimizes total energyE with respect to structure under the constraint of constant volumeV. Theminima of E are followed on successive sweeps of lattice parameters for 11 of 14 Bravais symmetries forone-atom-per-cell structures. The structures include the four orthorhombic phases.Also included are the hexagonal close-packed and cubic diamond phases withtwo atoms per primitive cell. No uniquely orthorhombic phases are found; allone-atom orthorhombic phases over a mega-bar pressure range are identical tohigher-symmetry phases. The simple cubic phase is shown to be stable where it is theground state. The number of distinct one-atom phases reduces to five plus thetwo two-atom phases. For each of these phases the Gibbs free energy at pressurep,G(p), is calculated for a non-vibrating lattice; the functionsG(p) give the groundstate at each p, the relative stabilities of all phases and the thermodynamic phase transition pressures forall phase transitions over a several-megabar range.