Transparent sodium under pressure

Matter contains multitudes. The list of chemical elements is impressive enough, a long list of substances with strange properties, even in isolation, even in “standard conditions” (normal pressure and temperature). But start fiddling with the variables, and you get nearly infinite combinations of phases and allotropes, different ways for the same atoms to clump together, all of which can have profoundly different tricks.

So perhaps it’s not surprising that we are still discovering new ways for matter to organize itself. Like transparent red sodium. Apply enough pressure to sodium, and it transmogrifies from its normal silvery white metallic self to a transparent red, um, “stuff.” Like a ruby.

Add some more pressure and the colour goes away, and it’s just a transparent solid — like diamond. Except made from a metal.

This funky allotrope of sodium was predicted and then partially confirmed in 2009. It is also an example of what is probably a general principle: other metals probably do the same thing when super-crushed. (Every single nerd reading this: “Transparent aluminum?” Yes, it has been achieved… for a few dozen quadrillionths of a second.)

Pics or it didn’t happen

When I first learned about red transparent sodium, I badly wanted to see something like a ruby brick, but I was disappointed. The problem, of course, is that we are talking about exotic pressures, the kind of super-crushing that can only be applied to microscopic samples, and even then only with great difficulty.

Which is also why no one noticed the phenomenon until 2009. Science’s big problem: too many variables, not enough time.

At a million atmospheres of pressure, sodium blackens (goth phase), and then at about 1.9 megabars it turns red (still pretty goth, actually). Those changes are tested and confirmed.

At 3 megabars, sodium is expected to go fully transparent… but no one has actually checked yet, because the experiment is just too tricky (insert can’t handle the pressure joke here).

Exotic pressures

A megabar is a million bars — a million times atmospheric pressure. Oof. At the bottom of the deepest canyons in the ocean, a kilobar is what you’re dealing with, and that can crush a submarine like an overripe raspberry. A megabar is a thousand times more intense.

Diamond forms at (very) roughly megabar pressures, but the diamond recipe also includes time and heat and other variables, so there’s no one diamond-making pressure.

The only place you can find multi-megabar pressures is in the cores of planets and stars, or for fractions of seconds when massive things smash into each other.

So we can’t make a brick of red transparent sodium, or even a chunk the size of the Star of Africa diamond (621g), or even a visible speck. But what a neat mental image.

Pile, D. Transparent sodium. Nature Photon 3, 250 (2009). https://doi.org/10.1038/nphoton.2009.51