Why do oxides form amorphous films instead of crystalline films?

Question

If I oxidize a metal at room temperature, say aluminum, why does it form an amorphous material instead of crystalline $\rm Al_2O_3$?

Answer 1

This is because the volume occupied by an Al2O3 molecule is different from that occupied by two Al's. This volumetric mismatch means that you can't retain the crystalline lattice of the Al across the interface where the Al ends and the Al2O3 begins. So you'll get a jumble of Al2O3 growing at that interface.

Here you have to know that an oxide film on aluminum grows in thickness when oxygen diffuses down through the oxide to reach the aluminum underneath it- and aluminum diffuses upwards through the oxide to reach the oxygen. In diffusing by randomly bouncing around through an oxide structure, the diffusing aluminum atoms "forget" what the crystal orientation of their original crystallite was and instead pop out of that glassy lattice to find oxygen at random spots in the growing film.

Remember too that a chunk of aluminum is itself not monocrystalline; it consists of billions of nanoscale "crystallites" with their crystallographic axes randomly oriented, so even without the mismatch issue the aluminum surface would form randomly-oriented bodies of oxide which would not be monocrystalline "in the bulk".

Now note that the Al-to-oxygen bonds are covalent, which means they possess directivity and so you cannot morph the bond angles effortlessly into any orientation you wish. This means that once the oxide starts growing amorphously it can't revert to a crystalline structure and winds up being glassy instead.

It is possible to grow monocrystalline Al203 by other means- just not by growing a film on a piece of solid aluminum.