We report molecular dynamics simulations of multiple radiation damage cascades in the pyrochlores Gd2Ti2O7 and Gd2Zr2O7 and in the solid solution Gd2(ZrxTi1-x)2O7 (x = 0.25, 0.50, 0.75). Using a simulation cell of approximately 360,000 atoms, for each compound 2,200 decay events are simulated over a total time of 10 ns, with each recoiling uranium atom (primary knock on atom, PKA) being assigned an initial kinetic energy of 5 keV. The structures generated in the simulations are analysed using Steinhardt local order parameters. There is a large increase in volume for the Ti pyrochlore associated with a transition to an amorphous structure which resembles the melt while preserving the immediate local environment of the Ti. The calculated dose for amorphisation is approximately 20 eV atom-1 which compares well with experiment. It appears to be the overlap of cascade and damage accumulation that drives the amorphisation and eventually suppresses the healing mechanisms. We have examined the variation in amorphous fraction with the number of decay events – an expression with two rather than one exponential terms reproduces the simulation data well. The behaviour of the zirconate is quite different – the substantial anion disorder produced by each recoil event is followed by healing between cascade events and reversion to the parent pyrochlore. In the solid solution the onset of amorphisation is delayed to successively later times on increasing the Zr concentration and the overall swelling reduced. Our simulations highlight the importance of ion mobility, associated with the weaker Zr-O bonds, in the healing process
|Publication status||Accepted/In press - 31 Jul 2020|
- molecular dynamics
- radiation damage
- nuclear waste
Archer, A., Foxhall, H. R., Allan, N. L., Shearer, J. R. W., Gunn, DSD., Harding, JH., Todorov, I., Travis, K. P., & Purton, JA. (Accepted/In press). Multiple Cascade Radiation Damage Simulations of Pyrochlore. Molecular Simulation.