Atomic-scale insights into electro-steric substitutional chemistry of cerium oxide

Haiwu Zhang, Ivano Eligio Castelli, Simone Santucci, Simone Sanna, Nini Pryds, Vincenzo Esposito*

*Corresponding author for this work

Research output: Contribution to journalJournal articleResearchpeer-review

Abstract

Cerium oxide is one of the most promising mixed ionic and electronic conducting material. Previous atomistic analysis has widely covered the effects of substitutional on oxygen vacancy migration. However, an in-depth analysis of the role of cation substitution beyond trivalent cations is rarely explored. Here, we investigate soluble monovalent (Li+, Na+, K+, Rb+), divalent (Fe2+, Co2+, Mn2+, Mg2+, Ni2+, Zn2+, Cd2+, Ca2+, Sr2+, Ba2+), trivalent (Al3+, Fe3+, Sc3+, In3+, Lu3+, Yb3+, Y3+, Er3+, Gd3+, Eu3+, Nd3+, Pr3+, La3+) and tetravalent (Si4+, Ge4+, Ti4+, Sn4+, Hf4+, Zr4+) cations substituents. By combining classical simulations and quantum mechanical calculations, we provide an insight into defect association energies between substituent cations and oxygen vacancies as well as their effects on diffusion mechanisms. Our simulations indicate that oxygen ionic diffusivity of subvalent cations substituted systems follows Gd3+>Ca2+>Na+. With the same charge, a larger size mismatch to Ce4+ cation yields a lower oxygen ionic diffusivity, i.e., Na+>K+, Ca2+>Fe2+, Gd3+>Al3+. Thanks to these trends, we then identify the species that could tune the oxygen ion diffusivity: we estimate the optimum oxygen vacancy site fraction (VO ••) to achieve fast oxygen ionic transport is 2.5% for GdxCe1-xO2-x/2, CaxCe1-xO2-x and NaxCe1-xO2-3x/2 at 800 K. Remarkably, such a concentration is not constant and shifts gradually to higher values with increasing temperature. We find that co-substitutions can enhance the impact of the single substitutions beyond their simple addition. Furthermore, we identify preferential oxygen ion migration pathways, which illustrates the electro-steric effects of substituent cations in determining the energy barrier of oxygen ion migration. Such fundamental insights into the factors that governing the oxygen diffusion coefficient and migration energy would enable designing criteria for tuning the ionic properties of the material, e.g., by co-doping.
Original languageEnglish
JournalPhysical Chemistry Chemical Physics
ISSN1463-9076
DOIs
Publication statusAccepted/In press - 2020

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