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.
