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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]

The effect of defects on the high‐pressure behavior of materials is fundamental to understanding and designing materials for extreme environments. Previous work has demonstrated that radiation damage can enhance phase stability or alter the transformation pathway. Here, we report the high‐pressure phase stability of CeO2 irradiated by swift heavy ions. CeO2 is of particular interest because it can be used as a non‐radioactive surrogate for UO2 and PuO2. High‐pressure Raman spectroscopy shows that low‐fluence ion irradiated CeO2 exhibits nearly the same high‐pressure behavior as unirradiated CeO2; whereas, after high‐fluence irradiation, the phase stability of CeO2 is significantly enhanced, with the critical phase transition pressure increased from 31.7 to 37.3 GPa and only about 7% high‐pressure phase transition fraction at 45.1 GPa. In comparison with the high‐pressure Raman results of pre‐irradiated CeO2 before and after 573, 1073, and 1 273 K annealing in air, we propose that large interstitial‐type defect clusters, such as dislocation loops formed in heavily‐irradiated CeO2, are predominately responsible for the increased phase stability at high pressures. [ABSTRACT FROM AUTHOR]