# What is chemical decomposition in the context of a crystal or amorphous solid?

The breakdown of a single entity (normal molecule, reaction intermediate, etc.) into two or more fragments.

That is a very general definition, but I am not sure if it is supposed to be this inclusive. For example, think of a diamond crystal. It is a single entity. Now if chemical decomposition occurs, it can break down into multiple fragments. These fragments are not defined more closely, so it could be that the fragments are single C atoms, polyatomic C molecules... or even diamond fragments again.

In this sense, simply cutting or polishing a diamond would be a special case of chemical decomposition. This seems quite absurd to me. So the question is: Is chemical decomposition really that broad, or is there some better definition that is more commonly used in chemistry? It seems to me that the intuitive definition of chemical decomposition would be that a molecule is broken into smaller fragments, while a crystal or an amorphous solid would need to be broken into non-crystalline (non-amorphous) fragments in order for it to be considered chemical decomposition. Otherwise it's just cutting!

The question is relevant to me because I am writing my PhD thesis on laser-induced breakdown spectroscopy. The "breakdown" in the name of the technique refers to a chemical breakdown, which is the same as chemical decomposition. Usually, solid samples are analyzed, in which case some material from the sample surface is removed by laser ablation. Laser ablation is basically the same as laser-induced breakdown, if you look at the definitions. So there is some confusion on the terminology in general, and I want to unravel it as good as possible in my theory chapter.

Thank you very much for your input!

(Note: By the way, some non-IUPAC definitions I have found for chemical decomposition also confusingly define it as relating to compounds. As far as I know, the definition of a compound excludes crystals and amorphous solids, but it also excludes homonuclear molecules. So this definition seems way too exclusive, because it makes sense that decomposition can also occur for homonuclear molecules.)

• It's decomposition if a relevant part of the sample (e.g. the one you actually analyse) is not the same substance as before. Polishing a diamond, you still have a diamond, plus some dust you don't care about much. – Karl Jan 7 '19 at 9:46

## 1 Answer

When it comes to non-molecular solids such as glass, metal or diamond, "a single entity" is in fact the formula unit, the concept identical to a "molecule" for, well, molecular compounds. Both formula unit and molecule are the smallest portion of a substance that preserves its chemical properties. As long as the formula unit remains the same, there is no chemical decomposition to speak of.

However, this doesn't mean that there is no chemical processes that can be initiated mechanically. An entire branch, mechanochemistry deals with them. As for laser ablation, I think you are basically introducing a secondary phase, a thin film of another substance (for example, metal oxide) on the surface.

One more practical example I can think of: for a single-crystal diffraction experiment one has to use a 3D crystal (in a sense that it shouldn't be a 2D plate for the proper absorpton correction) with linear dimensions not exceeding fractions of a millimeter. For the probe preparation a crystallographer often has to literally cut the crystals with a special scalpel under the microscope. However, the resulting crystal structure doesn't depend on how many cuts and in what direction the crystallographer did. The x-rays though, can alter the structure of some sensitive molecular crystals (often biological samples, proteins), but this is another story.

• Thank you, this sounds very reasonable and is something I had not thought of. The definition of a chemical entity is not given by IUPAC, but your interpretation as only relating to the formula unit probably makes the most sense. One unrelated question about the "phase" - I understand it correctly that a mixture of NaCl and KCl, for example, has a NaCl phase and a KCl phase? Or does a phase need to be physically connected so that a homogeneous mixture in fact contains hundreds of small NaCl and KCl phases? – PoorYorick Jan 5 '19 at 12:49
• This depends on what mixture you are talking about. A mechanical mix of both salts would indeed have 2 phases. However, a system of molten salts with an arbitrarily chosen composition $\ce{Na_xK_{1-x}Cl}$ ($0\leq x\leq 1$) is homogeneous all right (single phase, see e.g. this NaCl-KCl phase diagram). – andselisk Jan 5 '19 at 13:00
• Thank you, this is much appreciated! Yes, I was talking about a mechanical mix, because I was wondering whether several independent grains of NaCl would be considered the same "phase" in a mechanical mixture with something else. (From your reply, I get that the answer is "yes".) – PoorYorick Jan 5 '19 at 13:16
• Yep, if you take table salt and pour the powder of potassium chloride in, then thoroughly mix them, it's still considered a two-phase system. However, if you increase the temperature enough (say, up to 700 °C, see the phase diagram) and cool the molten salts down, you form a new chemical compound (single phase). This is not always the case and you always have to check the phase diagram first. – andselisk Jan 5 '19 at 13:23