Hafnium-containing compounds are of great importance to the semiconductor industry as a high-κ gate dielectric to replace silicon oxynitrides. Here, the crystallization processes and chemistry of bulk hafnia powders are investigated, which will aid in interpretation of reactions and crystallization events occurring in thin films used as gate dielectrics. Amorphous hafnia powder was prepared via a sol–gel route using the precursor HfOCl2·H2O. The powders were subjected to various heat treatments and analyzed using X-ray diffraction and thermal analysis techniques. A large change in the crystallization pathway was found to occur when the sample was heated in an inert environment compared with air. Instead of the expected monoclinic phase, tetragonal hafnia also formed under these conditions and was observed up to temperatures of ∼760°C. The tetragonal particles eventually transform into monoclinic hafnia on further heating. Possible mechanisms for the crystallization of tetragonal hafnia are discussed. It is proposed that, in an inert environment, tetragonal hafnia is stabilized due to the presence of oxygen vacancies, formed by the reduction of HfIV to HfIII. As the temperature increases the crystal grows until there are too few oxygen vacancies left in the structure to continue stabilizing the tetragonal phase, and hence transformation to monoclinic hafnia occurs.