Liquid-like phase formation in $Gd2Zr2O7$ by extremely ionizing irradiation

Isometric $Gd2Zr2O7$ with the ordered, pyrochlore structure has an extremely high resistance to radiation-induced amorphization. Ion-beam irradiations at keV to GeV energies result in a disordered, defect-fluorite structure that remains crystalline to very large fluences. However, we report liquid-like phase formation of droplet-like surface hillocks and quenched molten tracks in $Gd2Zr2O7$⁠. The extremely high energy density of 12-MeV $C60$ clusters creates tracks with substantial volumes of amorphous material, accompanied by the formation of nanocrystals of the disordered, defect-fluorite structure. This is the first evidence of irradiation-induced amorphization of $Gd2Zr2O7$⁠. In contrast, irradiation of $Gd2Zr2O7$ pyrochlore with swift heavy ions of U resulted in an order-disorder transformation to defect-fluorite without any evidence of amorphization. Thermal-spike calculations highlight the dominance of the effect of deposited energy density, controlled by the projectile velocity, as compared with the energy loss.