Phase stability, electronic structures and elastic properties of (U, Np)O2 and (Th, Np)O2 mixed oxides. Issue 27 (29th June 2018)

• Record Type: Journal Article
• Authors: Ghosh, P. S.; Kuganathan, N.; Arya, A.; Grimes, R. W.
• Is Part Of: Physical chemistry chemical physics. Volume 20:Issue 27(2018)
• Page Start: 18707
• Page End: 18717
• Publication Date: 2018-06-29
• Subjects: Chemistry, Physical and theoretical -- Periodicals; 541.3
• DOI: 10.1039/c8cp02414f
• Languages: English
• ISSNs: 1463-9076

Abstract

Abstract : Mixing enthalpies (Δ H mix ) of U1− x Np x O2 and Th1− x Np x O2 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 ). Abstract : Mixing enthalpies (Δ H mix ) of U1− x Np x O2 and Th1− x Np x O2 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 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 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 UO2 and ThO2 is employed to calculate Δ H mix of U1− x Np x O2 and Th1− x Np x O2 mixed oxides (MOX). The effect of collinear FM and AFM ordering is also considered in determining the Δ H mix . The calculated Δ H mix of Th1− x Np x O2 MOX were positive compared to the end members and nearly symmetric around x = 0.5 and Δ H mix of the AFM configuration were higher compared to the FM configuration maximum by 0.19 kJ mol −1 . The Δ H mix of U1− x Np x O2 MOX were negative up to U0.50 Np0.50 O2 with a maximum value of −1.21 kJ mol −1 for U0.4375 Np0.5625 O2 whereas Np-rich (U, Np)O2 MOX compositions exhibited Δ H mix close to zero. Values of Δ H mix 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− x Np x O2 and Th1− x Np x O2 MOX systems.