Nonequilibrium Phase Formation in Oxides Prepared at Low Temperature: Fergusonite‐Related Phases

Sol–gel methods have been developed to prepare YNbO 4 , YTaO 4 , and other rare‐earth niobates and tantalates with fergusonite‐related crystal structures. At low temperatures, all of the fergusonites, with the exception of SmTaO 4 , crystallize in a metastable tetragonal (T′) structure similar to that of tetragonal zirconia. Although all of the equilibrium forms of these oxides adopt a crystal structure containing an ordered distribution of the trivalent and pentavalent cations, a random cation distribution is obtained in the metastable T’phase. Metastable phase formation is often ascribed solely to kinetically limited topotactic crystallization. However, the changes in the grain size and unit‐cell volumes that accompany the metastable‐to‐equilibrium fergusonite conversions imply that other physical phenomena induced by small‐particle synthesis, namely the Gibbs‐Thompson pressure effect and the increased contribution of surface energy, cannot be ignored.