Structural and Electronic Properties of Po under Hydrostatic Pressure

Although Polonium (Po) is the only element of the periodic table that has a simple cubic (sc) crystal structure at ambient pressure, it is one of the least-studied elements, and its phase diagram is still unknown. Thus, with the aim to contribute to the study of the Po phase diagram, we present in this work theoretical calculations focused on determining the structural and electronic properties of Po under hydrostatic pressure. For that purpose, our theoretical study considers the sc structure as well as the hypothetical rhombohedral (r), body-centered cubic (bcc) and face-centered cubic (fcc) crystal structures. The calculations were performed using the full potential linearized augmented plane wave (FLAPW) method by using the local density approximation (LDA) for the exchange-correlation energy and by including the spin-orbit coupling to take into account relativistic effects. The total energy results were fitted to the third-order Birch-Murnaghan equation of state (EOS) for pressures up to 100GPa. Consequently, the energy results, along with the enthalpy findings, indicate that phase transitions follow the sequence sc -> r -> bcc -> fcc at pressure values of 2.47, 5.75, and 70.27GPa, respectively. These results are consistent with the fact that the hydrostatic pressure induces a change in the atomic distance and in the orbital hybridization that leads to different crystal structures.