Stable group-IIB elements —Zn, Cd and Hg at terapascal pressures

Abstract

The group-IIB elements, including zinc (Zn), cadmium (Cd), and mercury (Hg) are studied at multi-TPa pressures using first-principle electronic structure calculations coupled to an efficient structural prediction method. The experimental structural transition sequence with increasing pressure is successfully reproduced, and most metallic elements structures, for instance, simple cubic (sc), body-centered cubic (bcc) and samarium-type (Sm-type) are not stable at terapascal pressures of the group-IIB elements. In addition, the high-pressure hexagonal-close-packed (hcp) phase of Zn, Cd and Hg, can remain up to 6 TPa, and double hexagonal-close-packed (dhcp) and faced-centered cubic (fcc) phases transitions have been observed in Zn and Cd. Hg transforms directly from hcp to fcc, with dhcp absent. The calculated results also show that the c/a ratios of Zn and Cd have a similar transformation, and Hg goes towards the ideal value of 1.633 for a hexagonal close packing at high pressures. The calculated electronic properties indicate that Zn are high-pressure electrides with some of the electrons localized at interstitial sites enclosed by the surrounding atoms. The structural changes from hcp to dhcp and fcc are associated with a dynamical instability, and the soft mode phase transition mainly corresponds to the phase transition mechanism.