Polonium Is The Only Element With Simple Cubic Crystal Structure

Polonium is the only element with a simple cubic crystal structure, and new theoretical work explains why that is. In a solid piece of polonium the atoms sit at the corners of a cubic unit cell and nowhere else (see figure).

Many other materials have more crowded structures. For instance, in a face centered cubic (fcc) structure, atoms (such as copper, gold, nickel, and iridium) sit at the corners of the cube and in the center of each face. In body centered cubic structure (bcc), atoms (such as potassium, sodium, iron, and tungsten) sit at the corners of the cube and in the very center of the cube. Only polonium has the simple cubic (SC) structure.

One reason the study of Po so difficult is that it is highly radioactive and spews forth decay products; indeed, polonium has more isotopes, 36, than any other element. Physicists at the Academy of Sciences in the Czech Republic have now produced the first detailed theoretical explanation for polonium's unique crystal structure: it is the result of the complicated interplay of relativistic effects which become important in such heavy atoms as polonium (element 84).

Specifically they have identified the so-called mass-velocity term (describing the relativistic increase in mass of electrons traveling with velocities comparable to the velocity of light) as the cause of the simple-cubic structure of polonium.

Another polonium oddity: its elastic anisotropy is greater than for any other solid. That is, it is about 10 times easier to deform a Po crystal along the direction diagonal to the consolidated cubic cells than it is to deform the crystal in a direction perpendicular to any of the cubic faces. According to Dominik Legut (legut@ipm.cz, +420-530229-461), this property results directly from the simple cubic structure of polonium.

Polonium is a hazardous element that appears in the air and soil and in such plants as tobacco, tea, and mushrooms. (Legut et al., Physical Review Letters, upcoming article; text available at Physics News Select.