If the Electronegativity difference between the two species (say A and B) is very large, then the reaction results in formation of a compound. On the other hand, if the species have similar electronegativity values, therefore two solid solutions type are possible., i.e., Ordered & Disordered Solid Solutions

Ordered Solid Solution: If species A is surrounded by n B atoms and species B is surrounded by n A atoms then the resultant long range order exhibits the Ordered Solution behaviour. Examples are Al-Cu

Disordered Solid solution:  The solid solution exhibits ordered structure upto few angstroms(short distances)