Correlation between Sm³⁺optical absorption and disorder build-up in swift heavy ion-irradiated CeO₂nanoparticles

Diffuse reflectance spectroscopy was used to study undoped and Sm-doped CeO₂nanoparticles prepared by sol-gel and ball-milling routes, and to probe radiation damage induced by swift heavy ions. Kubelka-Munk analysis of UV-vis-NIR spectra reveals nine Sm³⁺4f-4f absorption bands within the wide O 2p-Ce 4f band gap, with intensities that increase with Sm content and remain broad owing to low local symmetry and native disorder. Under 946 MeV Au irradiation up to 5 × 10¹²cm^-2, ball-milled Sm-doped samples exhibit a systematic fluence-dependent decrease of Sm³⁺band intensities, without significant changes in bandwidth or peak positions, indicating that irradiation primarily weakens the local crystal field rather than altering site multiplicity. In parallel, the Urbach energy of the absorption edge increases, providing an optical measure of disorder build-up and yielding approximate damage cross sections consistent with electronically driven track formation. A weak Ce³⁺4f-5 d band near 15 000 cm^-1appears in irradiated undoped CeO₂but is absent in Sm-doped samples, in agreement with oxygen-potential pinning by charge-compensating vacancies. Overall, the data indicate that Sm³⁺absorption bands are sensitive optical probes of irradiation-induced disorder in nanocrystalline ceria and highlight the utility of rare-earth dopants for tracking radiation damage in fluorite-type oxide nanoparticles.