First-principles computational study of defect clustering in solid solutions of ThO$_{2}$ with trivalent oxides

TL;DR

This study uses density functional theory to analyze defect clustering and energetics in ThO₂ solid solutions doped with various trivalent oxides, revealing stable defect patterns and implications for ionic conductivity.

Contribution

First-principles DFT calculations identify defect clustering patterns and energetics in ThO₂ doped with trivalent oxides, predicting high conductivity in La-doped solutions.

Findings

Stable defect clusters involve repulsive interactions between vacancies.

Formation enthalpies decrease with smaller, less electronegative dopants.

La-doped ThO₂ likely exhibits higher ionic conductivity.

Abstract

The energetics of mixing and defect ordering in solid solutions of fluorite-structured ThO$_{2}$ with oxides of trivalent cations (Sc, In, Y, Nd, La) are investigated by electronic density-functional-theory (DFT). Through DFT calculations of structures enumerated by lattice-algebra techniques, we identify the lowest-energy patterns for defect clustering for four separate dopant concentrations. The most stable structures are characterized by a repulsive interaction between nearest-neighbor vacancies on the oxygen sublattice. The enthalpies of formation with respect to constituent oxides are positive for all dopants considered, and show a tendency to decrease in magnitude as the size and electronegativity of the trivalent dopant decrease. Due to the small positive formation enthalpies and low oxygen-vacancy binding energy with La dopants, La$_{2}$O$_{3}$-ThO$_{2}$ solid solutions are predicted to have relatively high ionic conductivities relative to those for the other aliovalent dopants considered. Our results are compared with those for the more widely studied ZrO$_{2}$ and CeO$_{2}$ fluorite-structured solid solutions with trivalent cations.