Atomistic simulations of UO2 – towards extended defects modelling in oxides

by Dr Kosa Monica

Wednesday 27 Jun 2018, 11:00 → 12:00 Europe/Zurich

Metal oxides represent a growing asset in many energy related industries, due to their chemical, physical, and electronic properties on one hand and their commercial appeal on the other. Metal oxides are massively produced for batteries for mobile and vehicular applications, they receive considerable attention as electrocatalysts for fuel cells and photovoltaic applications, and notably, actinide oxides are used as nuclear fuels for massive energy production. During their operation cycles, materials undergo severe pressure and temperature changes forming plastic deformations. It is therefore of a key importance to understand their behaviour in the plastic regime for their favourable practical utilisation. Under plastic deformation conditions, extended defects, i.e. dislocations or grain boundaries are formed, affecting the performance traits such as, for example, electric charge conduction, ionic conduction, host atoms aggregation. Notably the nuclear fuel material uranium dioxide, UO2, undergoes severe microstructural changes along its fuel cycle, forming dislocations. These in turn affect fission gas product accumulation and transport, the thermal conductivity of UO2, directly affecting fuel’s safety. Experimental characterization of structural deformations in UO2 is difficult due to safety and cost of handling radioactive materials, making computer simulations markedly vital to bridge this gap. Here we present an atomistic simulations of -surface modelling for studying extended defects in UO2 using both Empirical Potentials (EP) and Density Functional Theory (DFT). The surface modelling concept entails dividing simulation cell into two halves, shifting one halve of the crystal with respect to another, thus partially mimicking slipping of two planes. The DFT computed -surfaces are qualitatively and quantitatively different from their EP counterparts. Electronic structure of UO2 during the plane slip suggests that f-orbital repopulation occurs during the slip, to accommodate the changing bonding interactions. Our DFT study emphasizes the importance of inclusion of proper electronic structure and bonding description for simulating behaviour in the plastic regime of oxides with complex electronic structure.