Ceramics destined for use in hostile environments such as nuclear reactors or waste immobilization must be highly durable and especially resistant to radiation damage effects. In particular, they must not be prone to amorphization or swelling. Few ceramics meet these criteria and much work has been devoted in recent years to identifying radiation-tolerant ceramics and the characteristics that promote radiation tolerance. Here, we examine trends in radiation damage behaviour for families of compounds related by crystal structure. Specifically, we consider oxides with structures related to the fluorite crystal structure. We demonstrate that improved amorphization resistance characteristics are to be found in compounds that have a natural tendency to accommodate lattice disorder.

Using first-principles calculations, we systematically predict the order-disorder energetics of series of zirconate ͑A 2 Zr 2 O 7 ͒, hafnate ͑A 2 Hf 2 O 7 ͒, titanate ͑A 2 Ti 2 O 7 ͒, and stannate ͑A 2 Sn 2 O 7 ͒ pyrochlores. The disordered defect-fluorite structure is modeled using an 88-atom two-sublattice special quasirandom structure ͑SQS͒ that closely reproduces the most relevant near-neighbor intrasublattice and intersublattice pair-correlation functions of the random mixture. The order-disorder transition temperatures of these pyrochlores estimated from our SQS calculations show overall good agreement with existing experiments. We confirm previous studies suggesting that the bonding in pyrochlores is not purely ionic and thus electronic effects also play a role in determining their disordering tendencies. Our results have important consequences for numerous applications, including nuclear waste forms and fast ion conductors.A 2 B 2 O 7 pyrochlores are an important class of oxide materials that are being considered for nuclear applications including the immobilization of actinides in nuclear waste 1-3 and inert matrix fuel. 4,5 The pyrochlore structure ͑space group Fd3m͒ is closely related to the fluorite structure ͑space group Fm3m͒ and can be considered as an ordered fluorite derivative, i.e., BO 2 fluorite with half of the tetravalent B 4+ cations replaced by trivalent A 3+ accompanied by chargecompensating structural oxygen vacancies. Here both the cations and anions are fully ordered such that each A 3+ ͑B 4+ ͒ cation is coordinated by eight ͑six͒ nearest-neighbor oxygen atoms. The pyrochlore structure can be transformed into the disordered defect-fluorite structure by randomly distributing the cations and anions on their respective sublattices.As a nuclear material, the host must be highly resistant to radiation damage. This is because irradiation will create atomic scale defects, which with time will accumulate, possibly leading to amorphization and/or microcracking of the crystalline structure. The two most important atomistic defects that are created in A 2 B 2 O 7 pyrochlore oxides when exposed to displacive radiation damage conditions are cation antisite defects ͑0 → A B + B A ͒ and anion Frenkel pairs ͑0 → V O +O i ͒. 6,7 Interestingly, these defects are precisely the ones that are responsible for the pyrochlore to fluorite orderdisorder transformation. In other words, radiation damage inherently involves disordering processes ͑for example, neutron-diffraction measurements showed that the Mg and Al sublattices in MgAl 2 O 4 spinel are fully disordered after high-fluence neutron irradiation 8 ͒. Our earlier studies 2,3 found that oxides that show a natural propensity to accommodate lattice disorder will be less susceptible to detrimental radiation damage effects such as amorphization. Therefore, the order-disorder phase transition under equilibrium condition is the focus of this work. An understanding of structural disorder is also important for other properties such as ionic conductivity. 9,10 A...

The detrimental effects of the fission gas Xe on the performance of oxide nuclear fuels are well known. However, less well known are the mechanisms that govern fission gas evolution. Here, in order to better understand bulk Xe behavior (diffusion mechanisms) in UO2±x we calculate the relevant activation energies using density functional theory (DFT) techniques. By analyzing a combination of Xe solution thermodynamics, migration barriers and the interaction of dissolved Xe atoms with U, we demonstrate that Xe diffusion predominantly occurs via a vacancy-mediated mechanism. Since Xe transport is closely related to diffusion of U vacancies, we have also studied the activation energy for this process. In order to best reproduce experimental data for the Xe and U activation energies, it is critical to consider the active charge-compensation mechanism for intrinsic defects in UO2±x. Due to the high thermodynamic cost of reducing U 4+ ions, any defect formation occurring at a fixed composition, i.e. no change in UO2±x stoichiometry, always avoids such reactions, which, for example, implies that the ground-state configuration of an O Frenkel pair in UO2 does not involve any explicit local reduction (oxidation) of U ions at the O vacancy (interstitial).

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