A comparison of radiation effects in crystalline ABO4-type phosphates and silicates

The effects of ion irradiation in the ABO4-type compounds were compared by performing experiments on four materials that include the most common crystal structures (monazite vs. zircon) and chemical compositions (phosphates vs. silicates) for these phases. Pure synthetic single crystals of ZrSiO4, monoclinic ThSiO4, LaPO4 and ScPO4 were irradiated using 800 keV Kr+ ions. Radiation damage accumulation was monitored as a function of temperature in situ in a transmission electron microscope. The activation energies for recrystallization during irradiation were calculated to be 3.1–3.3 eV for the orthosilicates but only 1.0–1.5 eV for the isostructural orthophosphates. For the ion-beam-irradiated samples, the critical temperature, above which the recrystallization processes are faster than damage accumulation and amorphization cannot be induced, is >700°C for ZrSiO4 but it is only 35°C for LaPO4. At temperatures above 600°C, zircon decomposed during irradiation into its component oxides (i.e. crystalline ZrO2 plus amorphous SiO2). The data are evaluated with respect to the proposed use of the orthophosphates and orthosilicates as host materials for the stabilization and disposal of high-level nuclear waste. The results show that zircon with 10 wt.% Pu would have to be maintained at temperatures in excess of 300°C in order to prevent it from becoming completely amorphous. In contrast, a similar analysis for the orthophosphates implies that monazite-based waste forms would not become amorphous or undergo a phase decomposition.