[zarabat]

I wonder why some materials like some of inorganic ones decompose before melting?

[opsomath]

There's nothing magic going on, it's just that some compounds have a decomposition temperature that's pretty low.

[Arkcon]

I suppose there is an answer to this question, but I'm betting its not at all simple.  You've got to figure the relative strengths of intermolecular binding energy (for say, melting) and the forced holding atoms together (for decomposition.)  With some examples, maybe you could begin to figure it out.  But its probably way more complicated than that for many compounds, considering that some atom may help the molecule decompose, by withdrawing electrons from another part of the molecule.  Or intermolecular interactions, one part of the molecule attacking itself, or part of another same molecule.  Probably, if you want to simplify the thought process, you might want to limit yourself to inorganic compounds.

[Corribus]

You've got to figure the relative strengths of intermolecular binding energy (for say, melting) and the forced holding atoms together (for decomposition.) 

And the associated changes in entropy...