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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 (I41/a$I{4}_{1}/a$, 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, (002¯)h∥(1¯10)s$(00\overline{2})_{\mathrm{h}}\Vert (\overline{1}10)_{\mathrm{s}}$//, and [010]h∥[1¯1¯1]s$[010]_{\mathrm{h}}\Vert [\overline{1}\;\overline{1}\;1]_{\mathrm{s}}$. 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 (I4¯2d$I{\overline{4}}_{2}d$) 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. [ABSTRACT FROM AUTHOR]

Although modifying high-pressure behavior through the particle-size of a material is well recognized, a clear mechanism, underlying the start and subsequent growth of a high-pressure phase, remains elusive. Here, we investigate the effects of particle-size on the formation of reidite (ZrSiO 4 , I 4 1 / a), a high-pressure phase of zircon (ZrSiO 4 , I 4 1 / amd), by comparing zircon nanoparticles with bulk. This is done by high pressure Raman scattering and synchrotron X-ray diffraction experiments on zircons pressurized in diamond anvil cells and subsequent transmission electron microscopy analysis of quenched samples. As compared to bulk, the larger surface energy of nanoparticles is the cause of a higher critical pressure required to start the transition to reidite. However, after passing the critical pressures, the similar growth trends between nanoparticles and bulk indicate that the reidite growth is dependent on the core, rather than the surface. These results are important to identify craters caused by meteorite impacts using reidite as a high-pressure indicator, as nanoparticles are formed in naturally-occurring, highly radiation-damaged zircon. • Zircon nanoparticles are more difficult to transfer to reidite than zircon bulk. • Nanoparticles with larger surface require a higher pressure to start the transition. • Nanoparticles and bulk show a similar growth trend above the starting pressure. • The core rather than the surface controls the growth of reidite. • HRTEM images evidence the coexistence of zircon and reidite. [ABSTRACT FROM AUTHOR]