Employing full-potential linearized augmented plane-wave method, we investigate the stability of Po in its ground-state simple cubic structure $\left(\alpha \text{-Po}\right)$ with respect to the trigonal spiral structure exhibited by Se and Te and to the displacive phase transformations into either tetragonal or trigonal phases. The origin of the phase stability of $\alpha \text{-Po}$ is analyzed with the help of densities of states, electronic band structures, and total energies of competing higher-energy structures corresponding to selected stationary points of the total energy. The electronic structures and total energies are calculated both within the generalized gradient approximation and local-density approximation (LDA) to the exchange-correlation energy as well as with and without inclusion of the spin-orbit (SO) coupling. The total energies are displayed in contour plots as functions of selected structural parameters and atomic volume. It turns out that the LDA calculation with SO interaction incorporated provides best agreement with existing experimental data and that the simple cubic structure of $\alpha \text{-Po}$ is stabilized by relativistic effects of core electrons. High elastic anisotropy of $\alpha \text{-Po}$ is explained as a consequence of its simple cubic structure and is compared with elastic properties of other crystal structures. Finally, an uniaxial tensile test for loading along the [001] and [111] directions is simulated; the corresponding theoretical tensile strengths calculated within the $\text{LDA}+\text{SO}$ approach amount to 4.2 GPa and 4.7 GPa, respectively, which are the lowest values predicted in an element so far. According to Pugh and Frantsevich criteria, $\alpha \text{-Po}$ is predicted to be ductile. Also a positive value of the Cauchy pressure confirms the metallic type of interatomic bonding.

• Received 16 September 2009

DOI:https://doi.org/10.1103/PhysRevB.81.214118