[andrewflnr]

You're assuming "unit cell" is a meaningful concept for these materials. Can x-ray diffraction completely characterize, say, a quasicrystalline structure? Moreover, even if you can tell what it's made of, that doesn't necessarily tell you how it was made or how it works.

[yk]

Well, there are other techniques for anything that is not a crystal, and they are similarly developed as x-ray diffraction, if nothing else, take an atomic force microscope and identify each individual atom directly.

> that doesn't necessarily tell you how it was made or how it works.

Absolutely, there may be materials were we don't have the slightest idea how they are made or how some property arises. But my point is, that the analysis techniques are at a level were it is pretty much guaranteed that an expression like "unknown alloy" is pretty much just clickbait, we know a great deal of anything that is made out of atoms a few days after a sample showed up in a laboratory.

[andrewflnr]

"Identify each atom individually" won't scale very far for complex materials. In short, I think it's unwarrantedly optimistic to assume that our current lab techniques, optimized for natural and human-fabricated materials, will immediately figure out (hypothetically) alien technology. No assumption is completely safe; even "made of atoms" is not guaranteed. Probable, but not guaranteed.

[krapp]

>even "made of atoms" is not guaranteed. Probable, but not guaranteed.

How is that not guaranteed?

[comicjk]

It is possible that there are other stable particles besides the ones we know now (like protons, neutrons, electrons, photons, neutrinos, etc). And maybe they can form stable arrangements which are not atoms, but which can still interact with ordinary matter. In that case you could have a material that was not made of atoms.

I don't think a lump of such material is in a warehouse in Nevada, but maybe it exists somewhere in the universe.

[krapp]

I would think those would just be made of different kinds of atoms but OK.

[andrewflnr]

An atom is a particular type of arrangement of protons, neutrons, and electrons. If something is not made of protons, neutrons, and electrons in that particular arrangement, it's not made of atoms, by definition.