Enzymes Can Cut or Glue Nanoparticle-Coated DNA
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Ab Initio Study of Polonium Reveals Origins of Simple Cubic Structure Polonium is a rare radioactive element that has potential for a variety of engineering applications from thermoelectric power generation to dust removal and elimination of static charges. Recently, polonium has been covered in news outlets after being found in cigarettes as well as some herbal teas, and because of its nefarious use in the poisoning of Alexander Litvinenko. Beyond its notoriety, polonium is unique among all elements as being the only elemental solid found with a simple cubic crystal structure. Researchers at the Institute of Physics of Materials and Masaryk University in the Czech Republic report on ab initio calculations that explain this unique structure as a result of the Darwin and mass-velocity relativistic interactions, but not spin-orbital coupling. As an intense alpha emitter, experimental exploration of polonium is difficult, but these simulations, appearing in the July 6 issue of Physical Review Letters (016402/1-4, DOI: 10.1103/PhysRevLett. 99.016402) describe new phases in pressure regimes that should encourage work with this difficult material. D. Legut, M. Friák, and M. Šob employ a local density approximation potential combined with relativistic effects to simulate polonium in equilibrium and stressed states. Energy profile calculations, seen in Figure 1, at increased pressures demonstrate a collapse of the energy minimum associated with simple cubic structure. These calculations express the energy as a function of the trigonal deformation path.
Figure 1. Local density approximation energy profiles for polonium at various stress levels. Ln c/a = 0.693 corresponds to a simple cubic structure, for a trigonal deformation path with c in the [111] direction and a along a perpendicular axis. At higher pressures, the simple cubic structure destabilizes, and minima are observed corresponding to trigonal formations. Reprinted with permission from D. Legut, M. Friák, and M. Šob, Phys. Rev. Lett. 99, 016402 (2007). © 2007 by the American Physical Society. 686
A bimodal state comprised of trigonal structures, similar to the high temperature β phase of polonium as well as the structures of isoelectronic tellurium and selenium, is predicted at pressures of 1–3 GPa. The researchers then investigated the equilibrium simple cubic structure by comparing electronic structure calculations between polonium and tellurium while varying the relativistic factors active in the calculation. The spin-orbital coupling was found to not be of critical importance to achieving simple cubic structure, but the other relativistic effects were. The re searchers said that the contributions from the mass-velocity term and the Darwin term, a quantum perturbation, grow larger more quickly than the spin-orbit term for larger elements in the same periodic group. These results provide a comprehensive explanation of the special combination of relativistic factors that combine to give polonium a simple cubic structure. Additionally, by predicting a bimodal ph
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