First-principles study of the energetics of charge and cation mixing in U_{1-x} Ce_x O_2

TL;DR

This study uses DFT+U calculations to investigate the energetics of charge and cation mixing in U_{1-x} Ce_x O_2, revealing how charge transfer and cation arrangements influence material stability and properties.

Contribution

It introduces a computational approach that captures multiple charge states and cation arrangements, providing new insights into the energetics of mixed uranium-cerium oxides.

Findings

Charge transfer increases mixing energy by 4-14 kJ/mol.

Charge transfer is energetically disfavored but entropically stabilized at high temperatures.

Structural and energetic properties depend strongly on cation and charge ordering.

Abstract

The formalism of electronic density-functional-theory, with Hubbard-U corrections (DFT+U), is employed in a computational study of the energetics of U_{1-x} Ce_x O_2 mixtures. The computational approach makes use of a procedure which facilitates convergence of the calculations to multiple self-consistent DFT+U solutions for a given cation arrangement, corresponding to different charge states for the U and Ce ions in several prototypical cation arrangements. Results indicate a significant dependence of the structural and energetic properties on the nature of both charge and cation ordering. With the effective Hubbard-U parameters that reproduce well the measured oxidation-reduction energies for urania and ceria, we find that charge transfer between U(IV) and Ce(IV) ions, leading to the formation of U(V) and Ce(III), gives rise to an increase in the mixing energy in the range of 4-14 kJ/mol of formula unit, depending on the nature of the cation ordering. The results suggest that although charge transfer between uranium and cerium ions is disfavored energetically, it is likely to be entropically stabilized at the high temperatures relevant to the processing and service of urania-based solid solutions.