A model is presented that simulates the growth of Na-beta "-Al2O3 single-crystal films from sapphire substrates in alkali atmospheres. The formation of beta "-Al2O3 occurs via Na-O insertion, alpha-Al2O3 oxygen lattice transition and Al3+ ion redistribution. Calculations are presented, based on the chemical and structural free energy changes and the stresses induced by the necessary structural mismatch. Oxygen diffusion is examined at 1235-1350 degrees C on the basis of the model proposed and the microstructure development explained.