The effect of doping on the mechanical properties of rare-earth oxides - an atomistic study

Rare-earth metal oxides, such as cerium oxide (ceria, CeO₂), exhibit strong absorption in the IR spectrum, particularly when engineered or doped appropriately, which make it indispensable in various technological applications, especially in the realm of optics. CeO₂ is often lightly doped with certain elements that modify its electronic structure, optical properties, and catalytic activity that can benefit specific applications. The specific choice of dopant and doping concentration depends on the desired outcome and the trade-offs with other material properties. This study systematically investigates how dopants and associated point defects influence the anisotropy in the fluorite lattice and thereby impact the mechanical properties of doped CeO₂. We performed classic MD simulations with Buckingham type potentials to simulate pure CeO₂ and CeO₂ doped with trivalent Y₂O₃, Gd₂O₃, and Eu₂O₃ (denoted as YO_1.5, GdO_1.5 and EuO_1.5) and tetravalent ZrO₂. Trivalent dopants inherently generate charge-compensating oxygen vacancies, producing local lattice distortions and decreasing atomic cohesion, resulting in the degradation of mechanical stiffness. In contrast, tetravalent dopants such as ZrO₂ substitute isovalently and therefore preserve oxygen stoichiometry. The absence of vacancies, combined with stronger metal-oxygen bonding, enhances lattice cohesion and sustains or improves elastic moduli. Thermodynamic analyses, based on calculated potential energy distributions around defects and average cohesive energy, were used to link bond strength and ionic interactions to mechanical properties. The role of dopant-vacancy clustering, local energy variation, and lattice distortion in mechanical softening and stress localization was also explored at the atomic scale. Spin-polarized density functional theory (DFT) calculations provide insights into electronic effects, including charge redistribution and lattice relaxation. Overall, our results indicate that doped systems with stronger ionic potentials and smaller lattice distortions exhibit greater mechanical integrity and offers a mechanistic understanding of how dopant characteristics and defects govern the mechanical stability of rare-earth-doped CeO₂.