M.J.D. Rushton

Predicted pyrochlore to fluorite disorder temperature for A2Zr2O7 compositions

In a previous publication the order–disorder pyrochlore to fluorite transformation temperatures for a series of A 2 Hf 2 O 7 pyrochlores were predicted [C.R. Stanek and R.W. Grimes: Prediction of rare-earth A 2 Hf 2 O 7 pyrochlore phases. J. Am. Ceram. Soc. 2002, 85, p. 2139]. This was facilitated by establishing a relationship between these temperatures and the energy required to introduce a specific defect structure into the perfect pyrochlore lattice. Here an equivalent relationship for A 2 Zr 2 O 7 pyrochlores was generated, and from this the disorder temperatures for a number of compositions including Eu 2 Zr 2 O 7 were predicted.

Thermophysical and anion diffusion properties of (Ux,Th1−x)O2

Using molecular dynamics, the thermophysical properties of the (Ux,Th1−x)O2 system have been investigated between 300 and 3600 K. The thermal dependence of lattice parameter, linear thermal expansion coefficient, enthalpy and specific heat at constant pressure is explained in terms of defect formation and diffusivity on the oxygen sublattice. Vegards law is approximately observed for solid solution thermal expansion below 2000 K. Different deviations from Vegards law above this temperature occur owing to the different temperatures at which the solid solutions undergo the superionic transition (2500–3300 K). Similarly, a spike in the specific heat, associated with the superionic transition, occurs at lower temperatures in solid solutions that have a high U content. Correspondingly, oxygen diffusivity is higher in pure UO2 than in pure ThO2. Furthermore, at temperatures below the superionic transition, oxygen mobility is notably higher in solid solutions than in the end members. Enhanced diffusivity is promoted by lower oxygen-defect enthalpies in (Ux,Th1−x)O2 solid solutions. Unlike in UO2 and ThO2, there is considerable variety of oxygen vacancy and oxygen interstitial sites in solid solutions generating a wide range of property values. Trends in the defect enthalpies are discussed in terms of composition and the lattice parameter of (Ux,Th1−x)O2.

Effect of A-site cation disorder on oxygen diffusion in perovskite-type Ba0.5Sr0.5Co1−xFexO2.5

Molecular dynamics simulations of the effect of A-site cation disorder on oxygen diffusion in (Ba0.5Sr0.5)CoO2.5, (Ba0.5Sr0.5)FeO2.5 and (Ba0.5Sr0.5)Co0.8Fe0.2O2.5 were conducted to understand the oxygen diffusion mechanism. The diffusion coefficients of oxygen were strongly dependent upon the degree of A-site Ba/Sr cation ordering. The oxygen diffusion coefficient decreased and the oxygen diffusion activation energy increased with Ba/Sr cation ordering in the alternating (001) layers of the perovskite structure. The ordering of Ba/Sr cations also caused oxygen/vacancy ordering. In particular, vacancy location in the oxygen layers parallel to the Ba-rich layers significantly increased oxygen diffusivity in BSCF-related materials.

Prediction and characterisation of radiation damage in fluorapatite

Molecular dynamics simulations, used in conjunction with a set of classical pair potentials, have been employed to examine simulated radiation damage cascades in the fluorapatite structure. Regions of damage have subsequently been assessed for their ability to recover and the effect that damage has on the important structural units defining the crystal structure, namely phosphate tetrahedra and calcium meta-prisms. Damage was considered by identifying how the phosphorous coordination environment changed during a collision cascade. This showed that PO4 units are substantially retained, with only a very small number of under or over coordinated phosphate units being observed, even at peak radiation damage. By comparison the damaged region of the material showed a marked change in the topology of the phosphate polyhedra, which polymerised to form chains up to seven units in length. Significantly, the fluorine channels characteristic of the fluorapatite structure and defined by the structures calcium meta-prisms stayed almost entirely intact throughout. This meant that the damaged region could be characterised as amorphous phosphate chains interlaced with regular features of the original undamaged apatite structure.

Surface dependent segregation of Y2O3 in t-ZrO2

Atomistic simulation techniques have been used to predict the preferential segregation of Y3+ ions to the (100), (101) and (110) surfaces of tetragonal zirconia (t-ZrO2). It is found that segregation energetics vary greatly between surfaces. In particular, dopant ions segregate to the top of the (101) surface. Conversely, although they also segregate towards the (100) and (110) surfaces, Y3+ becomes trapped just beneath these surfaces. For all of these surfaces, segregation effects are negligible below 12Å. The surface orientation dependence will result in significant variations in the concentration of yttrium at different surfaces. As a consequence, properties that are a function of defect concentration and distribution will be surface dependent. Predictive understanding of such segregation effects will provide the possibility of better engineered devices for a variety of thermal and electrochemical applications.

Development of Xe and Kr empirical potentials for CeO2, ThO2, UO2 and PuO2, combining DFT with high temperature MD

The development of embedded atom method (EAM) many-body potentials for actinide oxides and associated mixed oxide (MOX) systems has motivated the development of a complementary parameter set for gas-actinide and gas-oxygen interactions. A comprehensive set of density functional theory (DFT) calculations were used to study Xe and Kr incorporation at a number of sites in CeO2, ThO2, UO2 and PuO2. These structures were used to fit a potential, which was used to generate molecular dynamics (MD) configurations incorporating Xe and Kr at 300 K, 1500 K, 3000 K and 5000 K. Subsequent matching to the forces predicted by DFT for these MD configurations was used to refine the potential set. This fitting approach ensured weighted fitting to configurations that are thermodynamically significant over a broad temperature range, while avoiding computationally expensive DFT-MD calculations. The resultant gas potentials were validated against DFT trapping energies and are suitable for simulating combinations of Xe and Kr in solid solutions of CeO2, ThO2, UO2 and PuO2, providing a powerful tool for the atomistic simulation of conventional nuclear reactor fuel UO2 as well as advanced MOX fuels.

Thermodynamic calculations of oxygen self-diffusion in mixed-oxide nuclear fuels

Mixed-oxide fuels containing uranium with thorium and/or plutonium may play an important part in future nuclear fuel cycles. There are, however, significantly less data available for these materials than conventional uranium dioxide fuel. In the present study, we employ molecular dynamics calculations to simulate the elastic properties and thermal expansivity of a range of mixed oxide compositions. These are then used to support equations of state and oxygen self-diffusion models to provide a self-consistent prediction of the behaviour of these mixed oxide fuels at arbitrary compositions.

From solid solution to cluster formation of Fe and Cr in α-Zr

To understand the mechanisms by which the re-solution of Fe and Cr additions increase the corrosion rate of irradiated Zr alloys, the solubility and clustering of Fe and Cr in model binary Zr alloys was investigated using a combination of experimental and modelling techniques — atom probe tomography (APT), x-ray diffraction (XRD), thermoelectric power (TEP) and density functional theory (DFT). Cr occupies both interstitial and substitutional sites in the α-Zr lattice; Fe favours interstitial sites, and a low-symmetry site that was not previously modelled is found to be the most favourable for Fe. Lattice expansion as a function of Fe and Cr content in the α-Zr matrix deviates from Vegards law and is strongly anisotropic for Fe additions, expanding the c-axis while contracting the a-axis. Matrix content of solutes cannot be reliably estimated from lattice parameter measurements, instead a combination of TEP and APT was employed. Defect clusters form at higher solution concentrations, which induce a smaller lattice strain compared to the dilute defects. In the presence of a Zr vacancy, all two-atom clusters are more soluble than individual point defects and as many as four Fe or three Cr atoms could be accommodated in a single Zr vacancy. The Zr vacancy is critical for the increased apparent solubility of defect clusters; the implications for irradiation induced microstructure changes in Zr alloys are discussed.

Migration of fluorine in fluorapatite – a concerted mechanism

Molecular dynamics simulations, used in conjunction with a set of classical pair potentials, have been employed to investigate the transport of fluorine in fluorapatite. A new coupled interstitial migration mechanism is identified with a migration activation energy of 0.55 eV in the temperature range 1100–1500 K. A full description of the mechanism is provided, which differs markedly from previously proposed vacancy mechanisms for fluorine transport.

A concerted mechanism for Cl− migration in chlorapatite

A highly anisotropic concerted vacancy mediated mechanism is identified for Cl− transport in chlorapatite. This was revealed in molecular dynamics simulations of stoichiometric and CaCl2 deficient chlorapatite. The mechanism was established within the temperature range 1000–1400 K, with an activation energy of 2.37 ± 0.07 eV. A considerably lower activation energy is predicted in the CaCl2 deficient material, 0.54 ± 0.16 eV, due to the availability of Cl− vacancies. The transport process involves the concerted migration of two to four Cl− ions directly along the c axis halide channel and is contrasted with the F− interstitial mechanism shown previously in fluorapatite.