Some elements go cubic under pressure

In the August 2007 issue of PHYSICS TODAY (page 24) a Physics Update item mentions recent work, by Dominik Legut and coworkers, proposing that the simple crystal structure of polonium arises from relativistic effects. The piece states that polonium is the only element with this structure. That is not true: For 20 years or more, the structure has been known to occur in phosphorus at pressures above 10 GPa, in calcium above 32 GPa, and in arsenic above 25 GPa. Since those elements are all much less heavy than polonium, it seems unlikely that relativistic effects can be sufficient to account for their simple cubic phases.

An item in the August 2007 Physics Update section states that "polonium, with atomic number 84, is the only element with a simple cubic crystal structure." In the 21st century, that statement is a bit extreme.

Selenium, in the same element group as polonium, has an allotrope¹ with simple cubic structure, as described in a text by Jerry Donohue.² Perusal of his book will also illumi-nate simple cubic structures of other elements.

References

1. 1. B. D. Sharma, J. Chem. Educ. 64, 404 (1987) [INSPEC].
2. 2. J. Donohue, The Structures of the Elements, Wiley, New York (1974), p. 385.

Legut replies: Richard Nelmes and Brahama Sharma are right that some other elements exhibit the simple cubic structure under high pressures and perhaps in thin films at elevated temperatures (see reference 1 in Sharma's letter). However, that point was not the goal of our research. When we wrote in our article that polonium is the only element with the simple cubic structure, we meant that it is the only element with that structure under ambient conditions. We hope readers of our article do understand that "under ambient conditions" is implied.

For phosphorus, calcium, and arsenic under high pressure, we doubt that their simple cubic structure would be due to relativistic effects. The atomic numbers of these elements are too low. Most probably, at high pressures, the Gibbs energy of the simple cubic phase becomes lower than the Gibbs energy of the original phase without involving relativistic effects.