The effect of relativity on stability of Copernicium phases, their electronic structure and mechanical properties

Abstract

The phase stability of the various crystalline structures of the super-heavy element Copernicium was determined based on the first-principles calculations with different levels of the relativistic effects. We utilized the Darwin term, mass-velocity, and spin-orbit interaction with the single electron framework of the density functional theory while treating the exchange and correlation effects using local density approximations. It is found that the spin-orbit coupling is the key component to stabilize the body-centered cubic ($bcc$) structure over the hexagonal closed packed ($hcp$) structure, which is in accord with Sol. Stat. Comm. 152 (2012) 530, but in contrast to Sol. Stat. Comm. 201 (2015) 88, Angew. Chem. 46 (2007) 1663, Handbook of Elemental Solids Z=104-112 (Springer 2015). It seems that the main role here is the correct description of the semi-core relativistic $6p_{1/2}$ orbitals. The all other investigated structures, i.e. face-centered cubic ($fcc$), simple cubic ($sc$) as well as rhombohedral ($rh$) structures are higher in energy. The criteria of mechanical stability were investigated based on the calculated elastic constants, identifying the phase instability of $fcc$ and $rh$ structures, but surprisingly confirm the stability of the energetically higher $sc$ structure. In addition, the pressure-induced structural transition between two stable $sc$ and $bcc$ phases has been detected. The ground-state $bcc$ structure exhibits the highest elastic anisotropy from single elements of the Periodic table. At last, we support the experimental findings that Copernicium is a metal.