COMPUTATIONAL AND EXPERIMENTAL STUDIES OF THE RADIATION RESPONSE OF Gd_2Ti_2O_7 PYROCHLORE

摘要

The structure and property changes in Gd_2Ti_2O_7 (polycrystalline pyrochlore) were examined following irradiation with 1 MeV Kr~+, 0.6 MeV Bi~+ and 4 MeV Au~(2+) ions over the temperature range 30-950 K. Gd_2Ti_2O_7 readily amorphizes with a low-temperature (30 K) critical dose for amorphization of ～0.15 displacements per atom (dpa). The critical temperature above which amorphization does not occur is about 1190 K. Nano-indentation studies reveal that the structural changes were accompanied by decreases of 15% in the Young's modulus. Subsequent 1 MeV Kr~+ irradiation of amorphous Gd_2Ti_2O_7 at 1065 K resulted in ion-beam-assisted recrystallization. These experimental studies were complemented with molecular dynamics simulations of low energy recoils in Gd_2Ti_2O_7 and Gd_2Zr_2O_7 using a Buckingham type potential. The displacement threshold energy surface in both pyrochlores is highly anisotropic. Displacement energies are higher for all sublattices in the titanate pyrochlore compared to the zirconate. Ti sublattice displacements require energies in excess of 100 eV, and result in multiple displacements and defect clusters. The formation of these clusters might impede dynamic defect recovery and facilitate amorphization.