  Vanadium oxides are strongly correlated materials which display
metal-insulator transitions as well as various structural and magnetic
properties that depend heavily on oxygen stoichiometry. Therefore, it is
crucial to precisely control oxygen stoichiometry in these materials,
especially in thin films. This work demonstrates a high-vacuum gas evolution
technique which allows for the modification of oxygen concentration in VOX thin
films by carefully tuning thermodynamic conditions. We were able to control the
evolution between VO2, V3O5, and V2O3 phases on sapphire substrates, overcoming
the narrow phase stability of adjacent Magn\'eli phases. A variety of annealing
routes were found to achieve the desired phases and eventually to control the
metal-insulator transition (MIT). The pronounced MIT of the transformed films
along with the detailed structural investigations based on x-ray diffraction
measurements and reciprocal space mapping show that optimal stoichiometry is
obtained and stabilized. Using this technique, we find that the thin film V-O
phase diagram differs from that of the bulk material due to strain and finite
size effects. Our study demonstrates new pathways to strategically tune the
oxygen stoichiometry in complex oxides and provides a roadmap for understanding
the phase stability of VOX thin films.
