ELASTIC INSTABILITY IN ION-BEAM-IRRADIATED MAGNESIUM ALUMINATE SPINEL

We present the first observation of an irradiation-induced instability in the shear modulus of MgAl2O4 spinel. Brillouin scattering was used to determine the elastic constant C-44 following 400 keV Xe2+ irradia tion of single-crystal spinel at 100 K. The corresponding structural c hanges were determined using electron microscopy. The results confirm a two-step transformation to the amorphous state through an intermedia te metastable crystalline phase. The shear elastic constant decreases by about 35% upon amorphization. This low-temperature radiation respon se, which is in contrast with the radiation resistance observed at ele vated temperatures, is discussued in terms of the ability of spinel to accommodate cation disorder and the free-energy difference between th e liquid and solid states.