### 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 |
---|---|

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 |

### Fingerprint

### ASJC Scopus subject areas

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

### Cite this

_{2}and (Th,Np)O

_{2}mixed oxides.

*Physical Chemistry Chemical Physics*,

*20*(27), 18707-18717. https://doi.org/10.1039/c8cp02414f

**Phase stability, electronic structures and elastic properties of (U,Np)O _{2} and (Th,Np)O_{2} mixed oxides.** / Ghosh, P. S.; Kuganathan, N.; Arya, A.; Grimes, R. W.

Research output: Contribution to journal › Article

_{2}and (Th,Np)O

_{2}mixed oxides'

*Physical Chemistry Chemical Physics*, vol. 20, no. 27, pp. 18707-18717. https://doi.org/10.1039/c8cp02414f

_{2}and (Th,Np)O

_{2}mixed oxides. Physical Chemistry Chemical Physics. 2018 Jun 20;20(27):18707-18717. https://doi.org/10.1039/c8cp02414f

}

TY - JOUR

T1 - Phase stability, electronic structures and elastic properties of (U,Np)O2 and (Th,Np)O2 mixed oxides

AU - Ghosh, P. S.

AU - Kuganathan, N.

AU - Arya, A.

AU - Grimes, R. W.

PY - 2018/6/20

Y1 - 2018/6/20

N2 - 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.

AB - 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.

UR - http://www.scopus.com/inward/record.url?scp=85049862132&partnerID=8YFLogxK

U2 - 10.1039/c8cp02414f

DO - 10.1039/c8cp02414f

M3 - Article

VL - 20

SP - 18707

EP - 18717

JO - Physical Chemistry Chemical Physics

JF - Physical Chemistry Chemical Physics

SN - 1463-9076

IS - 27

ER -