The March 12th issue of the journal Nature features one of the most interesting articles to date, in which researchers from Stony Brook University (SBU) and Jilin University (JU) show that the element sodium (chemical symbol Na) can become transparent when subjected to high pressures. This behavior was thought to be impossible on the part of substances belonging to the metal group, with which sodium is aligned. Now, the international team has proven that extreme amounts of pressure can indeed render the element transparent, a find that has potential applications in many areas of technology.
“It is well known that at sufficiently high compression all materials must go metallic. This is seen in the metallization of hydrogen at high pressures and temperatures inside planets Jupiter and Saturn. This fundamental result is important for understanding properties of highly compressed matter, particularly within stars and giant planets,” explains SBU professor of Theoretical Crystallography Artem Oganov, who led the new study with colleague Yanming Ma, who is a professor of physics at JU.
Knowing these things, it would then stand to reason that the element sodium behaves exactly the same, as it turns into metal at very high pressures. But experiments performed in the lab proved the exact opposite. When subjected to ever-increasing pressures, the material first turns black, and then red-transparent, at 2 million atmospheres. In nature, Na occurs as a gray metal, but eventually, at pressures that are unfathomable, it turns into a compound that is just as transparent as glass.
“What fascinated us most is that the pressures at which this transformation was predicted were experimentally reachable, and that at these conditions such a remarkable change of chemistry occurs,” Oganov says. The researchers theorized that, at high pressures, the very structure of the metal would change, and that the pressing force would cause the overlapping sodium atoms to shift their electrons in the tiny “holes” that occur between the atoms themselves.
“In these holes electrons demonstrate an extremely localized behavior, responsible for the collapse of the metallic state. These electrons behave as ‘fake atoms,’ just like in electrides – ionic compounds where the role of the anion is played by localized electrons,” Ma explains. It's yet unclear how scientists around the world would use this new find, but, most likely, other metals will be tested for similar behaviors.