Science Daily is reporting that a team led by Artem Oganov (Stony Brook University) and Yanming Ma (Jilin University in China) have discovered something pretty cool: at pressures of about 3 million atmospheres, sodium becomes transparent.
Normally, at room temperature and pressure, sodium is an opaque, whitish metal. At higher pressure, over a million times the Earth's atmospheric pressure, it turns black. And at 1.9 million atmospheres it starts to becomes transparent at red wavelengths. Although it hasn't been directly shown, the scientists expect at 3 million atmospheres it will becomes as transparent as glass.
The reason this is so interesting is that, for one, it wasn't predicted until recently. No one knew what would happen to sodium under such pressures. I read the explanation of what's going on at the link above, and as I understand it metals are opaque because they have electrons that are free to flow around the atoms in the metal. Electrons are very good at absorbing light, so metals are opaque (as well as good conductors due to the free electrons). At very high pressures, the electrons get locked up in spaces between the atoms and cannot move. This allows light to pass through the material (and I assume makes them insulators as well).
Another interesting aspect of this is that pressures like this are common deep inside planets. How heat gets transferred out from the cores of planets depends in part on how well light can pass through the materials, so anything that furthers our knowledge of this will help understand how planets work.
Sodium is not a material that, by itself, can be used in construction and so on, like, say, aluminum. But how far are we from proving Scotty right?