22
$\begingroup$

This question on NaCl crystalization actually got me wondering: are there any ionic amorphous solids? Like ionic crystals are crystalline materials of electrostatically-attracted ions, can ions form an amorphous phase? I can see no reason why not, but I cannot think of any example either…

$\endgroup$
  • $\begingroup$ Do you mean are there any that will naturally form, or are there ways to take substances that would normally form crystalline solids and make them form amorphous solids instead (second is definitely possible) $\endgroup$ – soandos Apr 26 '12 at 1:12
  • $\begingroup$ @soandos examples of both would be nice… but I was thinking of the first. In fact, I think I do already have an example of the second, as you sure can freeze ionic liquids into a glassy amorphous state… $\endgroup$ – F'x Apr 26 '12 at 1:15
  • $\begingroup$ that is along the lines of what I was thinking. I would tend to doubt the existence of the first type. $\endgroup$ – soandos Apr 26 '12 at 1:17
11
$\begingroup$

I assume you meant amorphous solid phases, since you can always melt any ionic compound to form an essentially amorphous liquid phase.

Googling "amorphous salts" brings up a fair number of results in the primary literature, such as this and this and this. The defining characteristic of such amorphous solid phases is that they exhibit no measurable melting transition, i.e. no latent heat of fusion can be measured in a calorimetry experiment. There also examples of amorphous semiconductor oxides and amorphous metal oxides and amorphous metals - feel free to search for these and see for yourself. So yes, they do exist, and in fact are of research interest.

$\endgroup$
11
$\begingroup$

The difficulty with creating an ionic bond glass is that creating true glasses depends on a combination of directional bonding between atoms (or molecules) and a rate of cooling that doesn't give those bonds time to move into optimal positions. The result is a bit like piling together sticky spheres that become locked before they can find an optimal stacking arrangement.

Pure ionic bonds in contrast are more like slippery spheres that are attracted towards each others' centers, but don't have any particular preferences about which parts of their surfaces make contact.

Given that, the most likely result by far from very rapid cooling of a molten ionic solid will be to produce micro or nanocrystalline ionic solids that may superficially resemble glasses, but which under X-ray or neutron diffraction crystallography would prove actually to have crystal structure at a very fine scale.

With that said, you nonetheless can come up with a recipe for trying to make a truly glassy ionic solid, especially since there's no such thing as a pure ionic bond. You would want to cool a very thin layer of molten salt at at an incredibly high rate, at least millions of degrees per second if I recall glassy metal cooling rates rightly. For ionic, you would also want the result to be pretty close to absolute zero, since whatever degree of "directional stickiness" you can get out of ionic solids is likely to be less even than metallic bonds.

Finally, if you really want to try to make such a thing as a serious project, you of course must check the literature in detail. My rule of thumb is that if you dig deep enough, there are almost no decently plausible ideas that you can think of in the physical sciences that someone has not at least written a paper on in some journal, and maybe even done some experiments.

Still, what you are asking is an intriguing idea for a real experiment, and it wasn't that many years ago that both metallic glasses and Penrose-tiled quasicrystals were new ideas.

$\endgroup$
  • $\begingroup$ On the contrary, there are numerous examples of amorphous salts, amorphous metal and amorphous metal oxides in the research literature that do not require cooling rates of "millions of degrees per second" to be formed. $\endgroup$ – Jiahao Chen May 12 '12 at 7:18
  • $\begingroup$ Examples welcome! Organic salts will be much easier, but if you can find a reference to a simple elemental salt that shows no micro or nano crystalline structure under X-ray diffraction ("amorphous" can be a bit ambiguous, and often includes such not-real-glass states), that would be especially interesting. $\endgroup$ – Terry Bollinger May 12 '12 at 8:41
  • $\begingroup$ Also, a "just in case" clarification: "millions of degrees per second" was just the unit used in some very early paper I read. The equivalent unit of "degrees per microsecond" captures the modest ranges a lot better. Such work used very thin films chilled very quickly indeed to keep the bonds cattywampus (technical term, sorry) down to the atomic level. Nanocrystals can be tough to prevent, so the earliest glassy metals were limited to very thin films. That got fixed as folks developed better alloys, ones that leave the atoms scratching their heads when deciding quickly who to bond to next. $\endgroup$ – Terry Bollinger May 14 '12 at 22:37
  • $\begingroup$ Just add lots of different ionic units with complicated ratios to form an intricate salt lattice and the cooling rate can be reduced to 1000C/s form a glass. After all that's the basis of metallic glasses. $\endgroup$ – user2617804 May 30 '14 at 12:52
4
$\begingroup$

The following is not a proof, but an idea that would imply that it is not possible to have such a material.

The reasoning goes as follows:

In any ionic "sea" of atoms, there are two types of situations:

  1. The atoms have enough energy to move freely, disregarding the attractive and repulsive forces of the other ions in the sea. This would cause a fluid, as the particles are not maintaining a position anywhere.
  2. They do not have that much energy, and therefore they will be impacted by the attractive and repulsive charges in the "sea." It then follows that there is an optimal configuration that will stabilize all of these forces. While I suppose that it might be possible for the optimal configuration not to be periodic (and thus not crystalline) I would tend to believe that it is not possible (it would seem from tessellations that if one assumes some basic characteristics, they must be regular). In a similar vein, I believe that it is not possible to have degrees of freedom in such an optimized state (though there may of course be more than one locally optimal state possible) as each atom is at a local minimum of energy, and it lacks the ability to "move up" the gradient (if it had more energy, it would be a liquid).

I therefore believe that it is improbable that such things exist.

$\endgroup$
  • $\begingroup$ By your argument, how do you explain the existence of a glass? As I'm sure you're aware (this is more for other people), the use of "proof" should never leave a chalkboard dedicated to mathematics. "Proof" does not belong in a physical science. $\endgroup$ – Chris Apr 26 '12 at 6:14
  • $\begingroup$ Glasses cool too quickly is my understanding. Yyou can create an amorphous ionic compound of any type by cooling it rapidly (see comment to question) $\endgroup$ – soandos Apr 26 '12 at 8:10
3
$\begingroup$

I would advise those who doubt in the existance of ionic solids to google ionic liquids which are widely commercialised. They probably solidify at sub-zero temperatures.

http://pubs.acs.org/doi/abs/10.1021/ie101653n?journalCode=iecred

Many pharmaceutical salts can be obtained in the solid amorphous state by spray-drying, freeze-drying or milling.

$\endgroup$
1
$\begingroup$

The textbook example are crystals isotropized over geologic timescales through the effect of ionizing radiation. Heat them up, to overcome the activation barrier, and they release the accumulated energy and turn crystalline again.

The relevant keyword is "Wigner effect".

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.