Structural phase stability in lithium to ultrahigh pressures

Abstract
The relative phase stabilities of the fcc, bcc, and hcp structures for Li are studied up to a compression of 100 by the linear muffin-tin-orbitals method. We make the first theoretical study of phase stability for Li that includes the hcp phase above 350 kbar and find a new high-pressure hcp phase to be stable between 5.4 and 26 Mbar. This phase results from a 2s-to-2p electronic transition that induces a series of phase transitions (fcc→hcp→bcc) in the ultrahigh-pressure range of 5.4 Mbar. Above 26 Mbar, the relatively open bcc structure remains the most stable phase up to the highest pressures that were examined (1000 Mbar). At low pressures, the two close-packed structures (fcc and hcp) are found to be favored over the bcc structure, which contradicts a recent calculation showing bcc stability at zero pressure.