  The epitaxy of MoO2 on c_plane sapphire substrates is examined. A theoretical
approach, based on density functional theory calculations of the strain energy,
allowed to predict the preferred layer/substrate epitaxial relationships. To
test the results of these calculations, MoO2/(001) Al2O3 heterostructures were
grown using the chemically_driven isothermal close space vapour transport
technique. At the early stages of the growth, two kinds of morphologies were
obtained, using the same growth parameters: lying and standing flakes. The
composition and morphology, as well as the layer/substrate epitaxial
relationships were determined for both kind of morphologies. Experimental
epitaxial relationships coincide with those predicted by DFT calculation as the
most favourable ones in terms of strain energy. For thicker films, the standing
flakes evolve to form an epitaxial porous layer composed by coalesced epitaxial
flakes. The interfacial strain between the sapphire substrate and MoO2 enables
a self_organization from nanometer to micron scales between separated or
coalesced flakes, depending on deposition condition.
