Transformation and clustering of defects induced by electron irradiation in barium hollandite ceramics for radioactive cesium storage:Electron paramagnetic resonance study

摘要

Barium hollandite-type ceramics have been envisaged for the immobilization of radioactive cesium. To evaluate their stability under irradiation, a hollandite ceramic of composition Ba_(1.16)Al_(2.32)Ti_(5.68)O_(16) was irradiated with electrons at a temperature close to room temperature to simulate the effect of β- and γ-decays of cesium. Ti~(3+) and O_2~- paramagnetic defects induced by electron irradiation [V. Aubin-Chevaldonnet et al., J. Phys.: Condens. Matter 18, 4007 (2006)] were detected by electron paramagnetic resonance. As the temperature in the bulk of the hollandite waste form could reach 300 ℃ at the beginning of the storage, the thermal stability of these paramagnetic defects was also studied. Isothermal annealing treatments at 300 ℃ and isochronal annealing treatments between 50 ℃ and 800 ℃ show that the irradiation induced Ti~(3+) (E_1 and E_2 centers) and O_2~- (H centers) do not recombine. Instead, they partially transform during annealing, respectively, into titanyl TiO~+ centers (E_3 centers) at the grain surface and into paramagnetic clusters of O_2 of less than 10 nm size (G_2 centers), trapped in the bulk of the grains. These oxygen-rich aggregates could prefigure the formation of molecular oxygen observed in electron irradiated glasses.