Why Is Polonium Simple Cubic and So Highly Anisotropic?

Using the state-of-the-art ab initio electronic structure calculations, we explain why $\alpha$-Po prefers the simple cubic structure (it is due to the relativistic mass-velocity and Darwin terms), elucidate its extreme elastic anisotropy (this is an intrinsic property of the simple cubic crystal structure), and predict a transformation to a mixture of two trigonal structures at pressures of 1–3 GPa.

Figure 1

The total energy profiles of Po calculated within the LDA with the SO coupling included along trigonal deformation paths for various volumes studied. The energies are given relative to the ground-state energy $E_0$. The full line with circles shows the total energy of Po at equilibrium atomic volume ($V/V_{\exp }=0.99$).

Figure 2

The energy-volume curves of Te and Po in their ground state and mutually reversed structures calculated when all relativistic effects were neglected. The trigonal spiral structures were fully relaxed.

Figure 3

The elastic anisotropy factor $A=C_{44}/C^{\prime }$ for sc Po as a function of pressure. Relativistic effects are included.