A transient liquid-like phase in the displacement cascades of zircon, hafnon and thorite

Abstract

The study of radiation effects in solids is important for the development of ‘radiation-resistant’ materials for fission-reactor applications1. The effects of heavy-ion irradiation in the isostructural orthosilicates zircon (ZrSiO4), hafnon (HfSiO4) and thorite (ThSiO4) are particularly important because these minerals are under active investigation for use as a waste form for plutonium-239 resulting from the dismantling of nuclear weapons2,3,4. During ion irradiation, localized ‘cascades’ of displaced atoms can form as a result of ballistic collisions in the target material, and the temperature inside these regions may for a short time exceed the bulk melting temperature. Whether these cascades do indeed generate a localized liquid state5,6,7,8 has, however, remained unclear. Here we investigate the irradiation-induced decomposition of zircon and hafnon, and find evidence for formation of a liquid-like state in the displacement cascades. Our results explain the frequent occurrence of ZrO2 in natural amorphous zircon9,10,11,12. Moreover, we conclude that zircon-based nuclear waste forms should be maintained within strict temperature limits, to avoid potentially detrimental irradiation-induced amorphization or phase decomposition of the zircon.