RESUMEN
Mixing enthalpies (ΔHmix) of U1-xNpxO2 and Th1-xNpxO2 solid solutions are derived from atomic scale simulations based on density functional theory (DFT) employing the generalised gradient approximation corrected with an effective Hubbard parameter (Ueff). The variation of structural and electronic properties of UO2 and NpO2 with collinear ferromagnetic (FM), collinear anti-ferromagnetic (AFM) and non-collinear anti-ferromagnetic arrangements of the uranium and neptunium magnetic moments are investigated while ramping up Ueff from 0 eV to 4 eV (the Ueff-ramping method). A combination of the Ueff-ramping method to treat the presence of metastable magnetic states and special-quasirandom structures (SQS) for the random distribution of Np atoms in UO2 and ThO2 is employed to calculate ΔHmix of U1-xNpxO2 and Th1-xNpxO2 mixed oxides (MOX). The effect of collinear FM and AFM ordering is also considered in determining the ΔHmix. The calculated ΔHmix of Th1-xNpxO2 MOX were positive compared to the end members and nearly symmetric around x = 0.5 and ΔHmix of the AFM configuration were higher compared to the FM configuration maximum by 0.19 kJ mol-1. The ΔHmix of U1-xNpxO2 MOX were negative up to U0.50Np0.50O2 with a maximum value of -1.21 kJ mol-1 for U0.4375Np0.5625O2 whereas Np-rich (U,Np)O2 MOX compositions exhibited ΔHmix close to zero. Values of ΔHmix for (Th,Np)O2 are consistent with a simple miscibility-gap phase diagram while those for (U,Np)O2 suggest more complex behaviour. Nevertheless, lattice parameter variation with composition still follows a Vegard's law relationship. Finally, single crystal elastic constants of pure oxides and MOX are reported. The linear-elasticity models describe the mixing energies to within an accuracy of approximately 1 kJ mol-1 for the U1-xNpxO2 and Th1-xNpxO2 MOX systems.
RESUMEN
This study reports the density functional theory (DFT) and classical molecular dynamics (MD) study of the lattice dynamical, mechanical and anionic transport behaviours of ThO2 in the superionic state. DFT calculations of phonon frequencies were performed at different levels of approximation as a function of isotropic dilation (ε) in the lattice parameter. With the expansion of the lattice parameter, there is a softening of B1u and Eu phonon modes at the X symmetry point of the Brillouin zone. As a result of the nonlinear decrease at the X point, the B1u and Eu phonon modes cross each other at ε = 0.03, which is associated with a sharp increase in the narrow peak of the phonon density of states, signifying a higher occupation and hence a higher coupling of these modes at high temperatures. The mode crossing also indicates anionic conductivity in the ã001ã direction leading to occupation of interstitial sites. Moreover, MD and nudged elastic band calculated diffusion barriers indicate that ã001ã is the easy direction for anion migration in the normal and superionic states. With a further increase in the lattice parameter, the B1u mode continues to soften and becomes imaginary at a strain (ε) of 0.036 corresponding to a temperature of 3430 K. The calculated temperature variation of single crystal elastic constants shows that the fluorite phase of ThO2 remains elastically stable up to the superionic regime, though the B1u phonon mode is imaginary in that state. This leads to anionic disorder at elevated temperatures. Tracking of anion positions in the superionic state as a function of time in MD simulations suggests a hopping model in which the oxygen ions migrate from one tetrahedral site to another via octahedral interstitial sites.
RESUMEN
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.
RESUMEN
The ability to tune the properties of graphene nanoribbons (GNRs) through modification of the nanoribbon's width and edge structure widens the potential applications of graphene in electronic devices. Although assembly of GNRs has been recently possible, current methods suffer from limited control of their atomic structure, or require the careful organization of precursors on atomically flat surfaces under ultra-high vacuum conditions. Here we demonstrate that a GNR can self-assemble from a random mixture of molecular precursors within a single-walled carbon nanotube, which ensures propagation of the nanoribbon in one dimension and determines its width. The sulphur-terminated dangling bonds of the GNR make these otherwise unstable nanoribbons thermodynamically viable over other forms of carbon. Electron microscopy reveals elliptical distortion of the nanotube, as well as helical twist and screw-like motion of the nanoribbon. These effects suggest novel ways of controlling the properties of these nanomaterials, such as the electronic band gap and the concentration of charge carriers.
