Soft Hydrothermal Synthesis of Hafnon, HfSiO₄

Despite being a member of the zircon-type silicate family, the conditions allowing the hydrothermal synthesis of HfSiO₄ are not well constrained. A multiparametric study was performed in order to follow the synthesis of this phase under soft hydrothermal conditions and thus to determine the most efficient conditions to form single phase samples. Among the experimental parameters investigated, concentration of reactants, pH of the reactive media, temperature and duration of the hydrothermal treatment impacted significantly the formation rate of hafnon and its crystallization state. Pure HfSiO₄ was obtained in acid reactive media with an acidity ranging from C_HCl = 1.5 M to pH = 1.0 and for C_Si ≈ C_Hf ≥ 0.21 mol·L^-1. The silicate phase was obtained after a 24 h treatment at temperatures ranging from 150 to 250 °C. However, the rise of temperature and extension of the duration of the hydrothermal treatment favored the crystallization state of the final HfSiO₄ samples.