The 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).