ABSTRACTThe temperature dependence of the critical amorphization dose, Dc, of four A2BO4 compositions, forsterite (Mg2SiO4), fayalite (Fe2SiO4), synthetic Mg2GeO4, and phenakite (Be2SiO4) was investigated by in situ TEM during 1.5 MeV Kr+ion beam irradiation at temperatures between 15 to 700 K. For the Mg- and Fe-compositions, the A-site is in octahedral coordination, and the structure is a derivative hep (Pbnm); for the Be-composition, the A- and B-sites are in tetrahedral coordination, forming corner-sharing hexagonal rings (R3). Although the Dc's were quite close at 15 K for all the four compositions (0.2–0.5 dpa), Dc increased with increasing irradiation temperature at different rates. The Dc-temperature curve is the result of competition between amorphization and dynamic recovery processes. The Dc rate of increase (highest to lowest) is: Be2SiO4, Mg2SiO4, Mg2GeO4, Fe2SiO4. At room temperature, Be2SiO4 amorphized at 1.55 dpa; Fe2SiO4, at only 0.22 dpa. Based on the Dc-temperature curves, the activation energy, Ea, of the dynamic recovery process and the critical temperature, Tc, above which complete amorphization does not occur are: 0.029, 0.047, 0.055 and 0.079 eV and 390, 550, 650 and 995 K for Be2SiO4, Mg2SiO4, Mg2GeO4 and Fe2SiO4, respectively. These results are explained in terms of the materials properties (e.g., bonding and thermodynamic stability) and cascade size which is a function of the density of the phases. Finally, we note the importance of increased amorphization cross-section, as a function of temperature (e.g., the low rate of increase of Dc with temperature for Fe2SiO4).
Dual Ion Beam Irradiation System for In situ Observation with Electron Microscope.
