Calculation of high-pressure phases of A1

Possible high-pressure phases of A1 are examined by comparing the total energies for three structures: fcc, bcc, and hcp. The energies are calculated using the density-functional formalism and the ab initio self-consistent pseudopotential approach. At zero pressure, the most stable structure is the fcc structure in agreement with experiment. At about 2 Mbar, the hcp structure is predicted to be the most stable, while at about 4 Mbar, the bcc structure is the most stable. The stability of the bcc phase at compressed volume is found to be caused by the lowering of the d states. This conclusion is based on an analysis of the occupancy of s, p, and d states at normal and compressed volumes.