Electron-irradiation- and ion-beam-induced amorphization of coesite

Electron-irradiation-induced amorphization in coesite was observed in situ as a function of temperature (15-750 K) at an incident electron energy of 1.0 MeV by transmission electron microscopy. Amorphization induced by ion-beam irradiation (1.5 MeV Kr-) in coesite was studied in situ as a function of temperature (15-875 K) using the high-voltage electron microscope (HVEM) Tandem Facility at Argonne National Laboratory. Electron-irradiation-induced amorphization in coesite was also observed at 200 keV and 300 K. Previously. the effect of temperature on amorphization was considered to only result in annealing, that is recovery of the damaged region. In this study, we show that temperature has an enhancing effect on amorphization and may even play a dominant role in the radiation-induced amorphization of some crystalline materials. The effect of temperature in enhancing: amorphization was first theoretically and experimentally recognized in electron- and ion-beam-irradiation-induced amorphization of coesite. Coesite has a melting temperature, T-m (875 K), below its glass transition temperature, T-g (1480 K). A thermodynamic analysis has been made to model the critical amorphization dose-temperature dependencies of electron- and ion-beam irradiations, We propose that the thermodynamic contributions to amorphization include the free-energy increase due to defect accumulation caused by irradiation and chemical disordering. Delta G(def) and Delta G(dis), and a thermal contribution, Delta G(therm). The thermodynamic condition for amorphization is generalized by the expression Delta G(total) = Delta G(therm) + Delta G(def) + Delta G(dis) greater than or equal to Delta G(am), where Delta G(total) is the total free-energy increase for the irradiated crystal, and Delta G(am) is the free-energy level required for solid-state amorphization. Coesite provides a good example of the dominance of the thermal contribution in amorphization. Solid-state amorphization of coesite induced by electron and ion irradiations is an irradiation-enhanced thermodynamic melting process below the glass transition temperature, T-g. The competition among thermally created dynamic displacements, irradiation-induced static displacements and the increased annealing rate at elevated temperatures, determines the critical amorphization dose-temperature dependencies of electron- and ion-irradiated coesite.