Hybrid Monte Carlo (HMC) simulations are used to study the growth of Gd-rich domains in Gd doped CeO2, and we probe the conductivity of the resulting and other configurations by molecular dynamics.

We apply bond order and topological methods to the problem of analysing the results of radiation damage cascade simulations in ceramics. Both modified Steinhardt local order and connectivity topology analysis techniques provide results that are both translationally and rotationally invariant and which do not rely on a particular choice of a reference structure. We illustrate the methods with new analyses of molecular dynamics simulations of single cascades in the pyrochlores Gd2Ti2O7 and Gd2Zr2O7 similar to those reported previously (Todorov et al 2006 J. Phys Condes. Mat. 18 2217).Results from the Steinhardt and topology analyses are consistent, while often providing complementary information, since the Steinhardt parameters are sensitive to changes in angular arrangement even when the overall topological connectivity is fixed. During the highly non-equilibrium conditions at the start of the cascade, both techniques reveal significant localised transient structural changes and variation in the cationconnectivity. After a few picoseconds, the connectivity is largely fixed, while the order parameters continue to change. In the zirconate there is a shift to the anion disordered system while in the titanate there is substantial reversion and healing back to the parent pyrochlore structure.3

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 1 . 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.

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