## Abstract

Mixing enthalpies (ΔH_{mix}) of U_{1-x}Np_{x}O_{2} and Th_{1-x}Np_{x}O_{2} 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 (U_{eff}). The variation of structural and electronic properties of UO_{2} and NpO_{2} 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 U_{eff} from 0 eV to 4 eV (the U_{eff}-ramping method). A combination of the U_{eff}-ramping method to treat the presence of metastable magnetic states and special-quasirandom structures (SQS) for the random distribution of Np atoms in UO_{2} and ThO_{2} is employed to calculate ΔH_{mix} of U_{1-x}Np_{x}O_{2} and Th_{1-x}Np_{x}O_{2} mixed oxides (MOX). The effect of collinear FM and AFM ordering is also considered in determining the ΔH_{mix}. The calculated ΔH_{mix} of Th_{1-x}Np_{x}O_{2} MOX were positive compared to the end members and nearly symmetric around x = 0.5 and ΔH_{mix} of the AFM configuration were higher compared to the FM configuration maximum by 0.19 kJ mol^{-1}. The ΔH_{mix} of U_{1-x}Np_{x}O_{2} MOX were negative up to U_{0.50}Np_{0.50}O_{2} with a maximum value of -1.21 kJ mol^{-1} for U_{0.4375}Np_{0.5625}O_{2} whereas Np-rich (U,Np)O_{2} MOX compositions exhibited ΔH_{mix} close to zero. Values of ΔH_{mix} for (Th,Np)O_{2} are consistent with a simple miscibility-gap phase diagram while those for (U,Np)O_{2} 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 U_{1-x}Np_{x}O_{2} and Th_{1-x}Np_{x}O_{2} MOX systems.

Original language | English |
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Pages (from-to) | 18707-18717 |

Number of pages | 11 |

Journal | Physical Chemistry Chemical Physics |

Volume | 20 |

Issue number | 27 |

DOIs | |

Publication status | Published - 20 Jun 2018 |

Externally published | Yes |

## ASJC Scopus subject areas

- Physics and Astronomy(all)
- Physical and Theoretical Chemistry

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