Soft hydrothermal synthesis of hafnon, HfSiO4

Despite being a member of the zircon type silicate family, the conditions allowing the hydrothermal synthesis of HfSiO4 were not well constrained. A multiparametric study was performed in order to follow the synthesis of this phase under soft hydro-thermal conditions and thus to determine the most efficient conditions to form single phase samples. Among the experi-mental 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 CHCl = 1.5 M to pH = 1.0 and for CSi ≈ CHf ≥ 0.21 mol·L 1. The silicate phase was prepared after a 24-hours treatment at temperatures ranging from 150°C to 250°C. However, rise of tem-perature and extension of the duration of the hydrothermal treatment favored the crystallization state of the final HfSiO4 samples.