Radiation damage and nanocrystal formation in uranium–niobium titanates

Two uranium–niobium titanates, U2.25Nb1.90Ti0.32O9.8 and Nb2.75U1.20Ti0.36O10, formed during the synthesis of brannnerite (UTi2O6), a minor phase in titanate-based ceramics investigated for plutonium immobilization. These uranium 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 room temperature were 4.72×1017 and 5×1017 ions/m2, respectively. At elevated temperature, the critical amorphization dose increases due to dynamic thermal annealing. The critical amorphization temperature for both Nb-rich and U-rich titanates is ∼933 K under a 800 keV Kr2+ irradiation. Above the critical amorphization temperature, nanocrystals with an average size of ∼15 nm were observed. The formation of nanocrystals is due to epitaxial recrystallization. At higher temperatures, an ion irradiation-induced nucleation-growth mechanism also contributes to the formation of nanocrystals.