A TRANSIENT LIQUID-LIKE PHASE IN THE DISPLACEMENT CASCADES OF ZIRCON,HAFNON AND THORITE

The study of radiation effects in solids is important for the developm ent of 'radiation-resistant' materials for fission-reactor application s'. The effects of heavy-ion irradiation in the isostructural orthosil icates zircon (ZrSiO4), hafnon (HfSiO4) and thorite (ThSiO4) are parti cularly important because these minerals are under active investigatio n for use as a waste form for plutonium-239 resulting from the dismant ling of nuclear weapons(2-4). During ion irradiation, localized 'casca des' of displaced atoms can form as a result of ballistic collisions i n the target material, and the temperature inside these regions may fo r a short time exceed the bulk melting temperature. Whether these casc ades do indeed generate a localized liquid state(5-8) has, however, re mained unclear. Here we investigate the irradiation-induced decomposit ion of zircon and hafnon, and find evidence for formation of a liquidl ike state in the displacement cascades. Our results explain the freque nt occurrence of ZrO2 in natural amorphous zircong(9-12) Moreover, we conclude that zircon-based nuclear waste forms should be maintained wi thin strict temperature Limits, to avoid potentially detrimental irrad iation-induced amorphization or phase decomposition of the zircon.