Radiation damage and nanocrystal formation in uranium-niobium titanates

Two uranium-niobium titanates, U2.25Nb1.90Ti0.32O9.8 and Nb2.75U1.20Ti0.36O 10, formed during the synthesis of brannnerite (UTi2O6), a minor phase in t itanate-based ceramics investigated for plutonium immobilization. These ura nium titanates were subjected to 800 keV Kr2+ irradiation from 30 to 973 K. The critical amorphization dose of the U-rich and Nb-rich titanates at roo m temperature were 4.72 x 10(17) and 5 x 10(17) ions/m', respectively. At e levated temperature, the critical amorphization dose increases due to dynam ic thermal annealing. The critical amorphization temperature for both Nb-ri ch and U-rich titanates is similar to 933 K under a 800 keV Kr2+ irradiatio n. Above the critical amorphization temperature, nanocrystals with an avera ge size of similar to 15 nm were observed. The formation of nanocrystals is due to epitaxial recrystallization. At higher temperatures, an ion irradia tion-induced nucleation-growth mechanism also contributes to the formation of nanocrystals. (C) 2001 Elsevier Science B.V. All rights reserved.