Polonium’s simple cubic structure

Polonium, with atomic number 84, is the only element with a simple cubic crystal structure, and new theoretical work by a team of scientists at the Academy of Sciences of the Czech Republic, Brno University of Technology, and Masaryk University explains why. In a solid piece of Po, the atoms sit at the corners of a cubic unit cell and nowhere else. Most other metallic elements, in contrast, have body-centered cubic, face-centered cubic, or hexagonally close-packed structure. Using state-of-the-art ab initio electronic structure calculations, the researchers have produced a detailed theoretical explanation for Po’s unique crystal structure: It is the result of the complicated interplay of relativistic effects that become important in heavy atoms such as Po. In particular, they found that the mass–velocity effect, which describes the relativistic increase in mass of electrons traveling at speeds comparable to that of light, increases faster than the effect of spin–orbit coupling in going down the periodic table from trigonal selenium and tellurium to Po. Another Po oddity: Its elastic anisotropy is greater than for any other solid. It is about 10 times easier to deform a Po crystal along the unit cell’s body diagonal (the [111] direction) than along a direction perpendicular to any of the cubic faces (the [100] direction). The team found that this property results directly from the simple cubic structure of Po. (D. Legut, M. Friák, M. Sob, Phys. Rev. Lett. 99 , 016402, 2007 http://dx.doi.org/10.1103/PhysRevLett.99.016402 .)