When is a crystal not a crystal?

I had thought I had about 3 months to write a paper, but it turns out iI have about 3 weeks. As you can imagine, this has kind of light a fire under my arse. This is especially true as the paper’s current status is: lots of kind of interesting looking data, no conclusion, no clear questions it attempts to either ask or answer, and no words down on paper. The data are on crystallisation. Crystallisation is how crystals form, for example how crystals of salt form when salt water evaporates.

Our results concern the fact that it may be harder to know when crystallisation has occurred than you, or I, would hope. Above is a schematic of part of a perfect salt crystal. A salt crystal is trillions of atoms of sodium and chloride ions arranged in a beautifully symmetric repeating lattice. If you look closely you can see that each sodium ion is surrounded by four chlorides, and vice versa.

This is great. And about a hundred years ago we realised that as the spacing between the ions was about the size of the wavelength of X rays, we could use these X-rays to probe the structure of crystals, and of the liquids they grow from. The pattern obtained from X-rays going through the salt crystal above is very different from that either molten salt, or salt dissolved in water. So, here X-rays allow us to tell how crystallisation is getting on. Very useful.

In the systems, a postdoc and I are studying, life is not so simple. In these systems, three different types of crystals can form (these are usually called polymorphs). In itself that would be fine, indeed, X-rays can easily distinguish between one crystal and another. However, our system doesn’t pick one of the three crystals, and just form that one, instead it forms a mish-mash of all three. The three are so mixed that an atom in a piece of one crystal type is maybe only 3 or 4 atoms from a piece of the one of the other two types of crystal. This mess of crystals mixture looks to X-rays not like a crystal but like the liquid.

Now, we are studying this via computer simulation, so we can see what it really looks like as we can track every atom. Experimentalists cannot do that – real atoms are very very small, which is why they often rely on X-rays. So for our system, an experimentalist relying on X-rays would see very little happening during crystallisation. The X-ray pattern would hardly change.

Indeed, in some sense it is questionable whether we are observing crystallisation at all. Can a state that has the X-ray pattern of a liquid actually be called a crystal? This kind of depends on what you mean a crystal. If by solid you mean a substance where the atoms or ions are fixed in a repeating lattice such they are regularly spaced over thousands or millions of atoms, then what we see is a not a crystal. But if by crystal you mean something where each atom is surrounded by a regular arrangement of, in our case eight or twelve, neighbours then we have a crystal.

I think this is interesting, but at the moment I guess the paper’s working title is: “Crystallisation via an intermediate-that-may-or-may-not-be-a-crystal-depending-on-how-picky-you-are”. Sadly, I can’t see that one flying. I need to work out what I mean by a crystal, why other people should about these funny crystals/not-crystals, and come with some sort of conclusion, all before Easter.