Phase stability, elasticity, and theoretical strength of polonium from first principles

Abstract

Employing full-potential linearized augmented plane-wave method, we investigate the stability of Po in its ground-state simple cubic structure ͑␣-Po͒ 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 ␣-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 ͑LDA͒ to the exchange-correlation energy as well as with and without inclusion of the spin-orbit ͑SO͒ 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 ␣-Po is stabilized by relativistic effects of core electrons. High elastic anisotropy of ␣-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 ͓001͔ and ͓111͔ directions is simulated; the corresponding theoretical tensile strengths calculated within the LDA+ 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, ␣-Po is predicted to be ductile. Also a positive value of the Cauchy pressure confirms the metallic type of interatomic bonding.

Structure and dynamics in amorphous tellurium and Tenclusters: A density functional study

“…The cubic form of Po (atomic number 84) is more stable than the trigonal form found in Se and Te due to relativistic effects. 25 The van der Waals radii (S: 1.85Å, Se: 2.00Å, Te: 2.20Å) 26 suggest that interchain bonds of length 3.3-3.5Å have a covalent component.…”

Phonon softening and superconductivity triggered by spin-orbit coupling in simple-cubicα-polonium crystals

“…As mentioned above, the Peierls instability is the key physics in Po [6,[10][11][12]. Even if the Peierls instability is suppressed by the SOC, α-Po in the sc phase would still have the soft phonon mode and so the strong electronphonon (EP) coupling.…”

“…The structural energetics in Po is known to be very sensitive to the volume and the utilized exchange-correlation method in the band calculation [8,10]. The lattice constant a exp of α-Po was first measured by Beamer et al [14] to be 3.345Å.…”

“…Recently, there have been several reports to explore the origin of the stabilized sc structure in Po [6][7][8][9][10][11]. General consensus so far is that the large relativistic effects in Po play an important role.…”

HgO at high pressures: the transition to the NaCl structure (HgO-III) and the equation of state of tetragonal HgO-II

“…Indeed, non-relativistic calculations predict that the NaCl structure should be stable in HgO at ambient pressures (Biering et al 2009). Thus, HgO is one of the few examples, along with the gold halides and molybdenum, in which relativistic effects have been proposed to control a non-actinide material's pressure-induced phase transitions (Boettjer 1999;Sohnel et al 2005;Kurzydlowski and Grochala 2008;Biering et al 2009;Legut et al 2010). Indeed, even at ambient pressure, relativistic effects have been proposed to account for *30% of the voltage associated with HgO-bearing mercury batteries (Zaleski-Ejgierd and Pyykkö 2011), so a primary role for such effects at high pressures is unsurprising.…”