Ion irradiation-induced amorphization and nano-crystal formation in garnets

The radiation susceptibility of garnet structure types (A₃B₂(XO₄)₃, I_a3d, Z = 8) has been examined by 1.0 MeV Kr²⁺ irradiation with in situ transmission electron microscopy over the temperature range of 50-1070 K. The target garnets included five natural garnets: almandine, andradite, pyrope, grossular and spessartine, and five synthetic garnets incorporating various contents of actinides. The synthetic garnets were silicates (N-series) and ferrate-aluminates (G-series). The critical amorphization temperature (T_c), above which amorphization does not occur, were determined to be 1050 K for N77, 1130 K for N56, 1100 K for G3, 890 K for G4 and 1030 K for andradite. T_c of the synthetic garnets increased with increasing mass density of the target and as the electronic-to-nuclear stopping power ratio decreased. During ion irradiation of the G3 garnet at a temperature of 1023 K near the T_c, nano-crystals were produced in which the unit-cell parameter of the original garnet was halved (∼0.64 nm) as a result of a radiation-induced recrystallization process, without evidence for ordering of the cations.