We know that Al tends to form amorphous oxide layer when exposed to atmosphere at room temperature. But what if the reaction of Al and O2 happen in very low pressure and high temperature? will the Al2O3 formed as crystalline?

http://en.wikipedia.org/wiki/Corundum

http://en.wikipedia.org/wiki/Sapphire

probably formed at high pressure. High temperature over a long time must be the key.

 

My gut feeling: very low pressure (together with heat) won't produce a crystalline oxide, because aluminium itself is polycrystalline, and because the number of Al atoms per area unit won't match that of the oxide. You have better chances starting with the molten oxide and achieve a directed solidification, by Czochralski or Bridgman methods.

http://en.wikipedia.org/wiki/Czochralski

http://en.wikipedia.org/wiki/Bridgman%E2%80%93Stockbarger_technique

which are common methods to produce single-crystals, including ceramics, for instance for lasers or for scintillators.

 

For tiny amounts, you might try epitaxy at low pressure, yes, but this needs a monocrystalline seed with a compatible lattice parameter. Then Al and O2 would be brought to the growing surface as gaseous molecules, and the by-products should be gaseous as well, and here troubles begin - choice is very narrow if any, leading semiconductor technology to use dangerous chemicals like SiH4, HF, AsH3...

"We know that Al tends to form amorphous oxide layer when exposed to atmosphere at room temperature."

No we don't.

I'm fairly sure I have read that the layer has a definite structure- albeit different to corundum. If my memory serves me well it's a defect rock salt structure.

According Housecroft's Inorganic Chemistry alpha-alumina (corrundum) displays a hexagonally closepacked lattice with oxide ions occupying two thirds of the octahedral interstitial sites. While gamma alumina has a defect spinel lattice.

 

Of course this goes for bulk alumina so I don't know how applicable this would be to an oxide layer on the surface of bulk Al(0).