# Why is Polonium the only metal to crystallize forming Simple Cubic Cell?

I have read that Polonium is the only metal that crystallizes forming SCC. I searched for a concrete explanation but all they were saying is the stability of Polonium at STP. I can't really comprehend their explanation.

• There is no simple explanation. If you think you have one, you are wrong. – Ivan Neretin Jun 22 '19 at 14:32
• (nit-pick: research is not a fancier version of "search", but means a carefully planned investigation, in which you expect to find something that is not yet known by others.) – Karl Jun 22 '19 at 19:43

1. The inorganic crystal structure database (ICSD), by specifying that the structure contains only a single element, you can reduce the number of hits to 55. Structure examples are Cr, Po, Mn, Sb, Se, P, Ca, Li, As. Some are specific phases occurring at different temperature or pressures.

So Po is not the ONLY metal to have a simple cubic structure, but the other cases require special conditions.

1. The origin of the stabilized simple-cubic (SC) structure in Po is explored by using the first-principles band calculations. We have found that the prime origin is the inherent strong spin-orbit (SO) interaction in Po, which suppresses the Peierls-type structural instability, as usually occurs in p-bonded systems. Based on the systematic analysis of electronic structures, charge densities, Fermi surfaces, and susceptibilities of Se, Te, and Po, we have proven that the stable crystal structure in VIA elements is determined by the competition between the SO splitting and the crystal-field splitting induced by the low-symmetry structural transition. Our study suggests that the large SO interaction would suppress the Peierls instability which is generally expected to occur in one-dimensional conductors.

2. In fact, Po becomes airborne with ease: if a sample is heated in air to 55 °C (131 °F), 50% of it is vaporized in 45 hours to form diatomic Po2 molecules. (The melting point of polonium is 254 °C and its boiling point is 962 °C.)

Putting it all together, Po is in the column with Se and Te, and just to the right of Bi, none of which are typical of "normal" metals. The formation of $$\ce{Po2}$$ molecules at near ambient temperatures suggests that the outer electrons of Po form bonds of a highly covalent nature.

But it is indeed unusual and fascinating, and if Po were not 5000 times more radioactive than radium, I might be tempted to do a little chemistry.

References

1. Quora: "What are examples of simple cubic crystal structure?" Gervais Chapuis, Nov 3, 2016

2. B. I. Min, J. H. Shim, Min Sik Park, Kyoo Kim, S. K. Kwon, and S. J. Youn, Origin of the stabilized simple-cubic structure in polonium: Spin-orbit interaction versus Peierls instability, Phys. Rev. B, 73, 132102 – Published 7 April 2006 https://journals.aps.org/prb/abstract/10.1103/PhysRevB.73.132102

3. https://en.wikipedia.org/wiki/Polonium