Phase transition of hafnon,HfSiO4, at high pressure

The high-pressure behavior of hafnon has been systematically investigated by combining in situ synchrotron X-ray diffraction, Raman, high-resolution transmission electron microscopy (HRTEM) techniques, and theoretical simulations. Hafnon starts phase transition at 26.6 GPa and completes the transition to an irreversible scheelite phase ($I{4}_1/a$, Z = 4, a₀ = 4.712 Å, and c₀ = 10.378 Å) at ∼45 GPa. The HRTEM observation of an interface between hafnon and scheelite phases allows atomic scale understanding of the transition process with a relationship of (200)_h‖(112)_s, ${(00\overline{2})}_{\mathrm{h}}\parallel {(\overline{1}10)}_{\mathrm{s}}$//, and ${[010]}_{\mathrm{h}}\parallel {[\overline{1}\overline{1}1]}_{\mathrm{s}}$. Hafnon shows a significantly lower transition pressure (∼12.6 GPa), as calculated from the relative enthalpies, than the measured pressure (∼26 GPa), indicating a kinetically hindered process involved in the transition. A high pressure low symmetry phase in hafnon ($I{\overline{4}}_{2}d$) is identified by the simultaneous appearance of two Raman modes (∼75 and 450 cm⁻¹) at 26.6 GPa and their subsequent simultaneous disappearance at 36.7 GPa. These results are important to understanding the mechanism of the zircon-scheelite transition for both zircon and hafnon.