The vanadium–oxygen system is very complex not only due to the multi-valences of vanadium but also contributing to the easy formation of a very wide range of ordered and disordered defect low equilibrium oxygen pressures and a narrow stability region. In Fig. 2, it was easily found that there were some peaks in different XRD patterns located at the same diffraction angle (2y). It also meant that even though the phases were unknown when the molar ratio was slightly departed from the proper value (higher or lower), there were some similar units with the same d-value in the structure of crystals of the final products.