A transmission electron microscopic study was made of the 1MeV electron irradiation-induced amorphization of coesite, as a function of temperatures of between 15 and 750K. The amorphization which was induced by 1.5MeV Kr+ bombardment of coesite was studied at temperatures ranging from 15 to 875K. Electron irradiation-induced amorphization in coesite was also observed at 200keV and 300K. It was recalled that the effect of temperature upon amorphization had previously been considered to result only in annealing (recovery of the damaged region). It was shown here that the temperature had an enhancing effect upon amorphization, and could even play a predominant role in the radiation-induced amorphization of some crystalline materials. The effect of temperature in enhancing amorphization was theoretically and experimentally considered with regard to the electron and ion beam-induced amorphization of coesite. This material had a melting point (875K) that was below its glass transition temperature (1480K). A thermodynamic analysis was made of the critical amorphization dose- and temperature-dependences upon electron irradiation and ion bombardment. It was proposed that the thermodynamic contributions to amorphization included a free-energy increase that was due to defect accumulation which was caused by irradiation and chemical disordering, plus a thermal contribution. The thermodynamic condition for amorphization was generalized by an expression which equated the total free-energy increase for the irradiated crystal to the sum of defect accumulation, thermal, and disordering energies. This total had to be greater than the solid-state amorphization energy. It was concluded that coesite provided a good example of the predominance of a thermal contribution to amorphization. The solid-state amorphization of coesite which was caused by electron and ion irradiation was considered to be an irradiation-enhanced thermodynamic melting process below the glass transition temperature.

W.L.Gong, L.M.Wang, R.C.Ewing, J.Zhang: Physical Review B, 1996, 54[6], 3800-8