ACCUMULATION OF STRUCTURAL DEFECTS IN ION-IRRADIATED CA2ND8(SIO4)6O2

Ion irradiations of the rare-earth orthosilicate, Ca2Nd8(SiO4)6O2, have been carried out using both alpha particles (emitted from a (PuO2)-Pu-238 source) and 3 MeV argon ions. The unit cell exhibits anisotropic expansion under irradiation, consistent with expectations based on the polyhedral connectivity within the structure. A least-squares analysis of the interatomic distances suggests that the unit-cell expansions are primarily due to changes in oxygen-oxygen distances and cation separations between neighboring polyhedra rather than to bonds within polyhedra. The irradiation-induced change in unit-cell volume is proportional to 1 - exp(-BD), where B is an annealing rate constant and D is the dose, in agreement with a model for the accumulation of isolated point defects in the structure. The volume expansion saturates at 2.56% and 1.40% for the alpha and argon irradiations, respectively. Analysis of the results suggests that a significant fraction of the defects produced in the argon-ion displacement cascades are lost to in-cascade recombination. Differential scanning calorimetry of powder irradiated with 3 MeV argon ions to 20 ions/nm2 reveals an exothermic recovery peak at 350-degrees-C with an activation energy of 1.3 +/- 0.1 eV and average stored energy release of 28.2 J/g. There is no evidence for amorphization of this material under alpha or argon irradiation.