Effect of defect states and oxygen vacancies on optical transitions due to Co2+ substitution in CeO2

Abstract

CeO2 has cubic fluorite structure which is modified due to oxygen content as well as external substituents. The oxidation state of Ce plays an important role in strain and related physical properties. Ce3+ being larger in size than the Ce4+ ion, one expects a change in band structure due to changes in bond length; substitution of Ce4+ by Co2+ in sol–gel prepared, homogeneous, single-phase Ce1−xCoxO2 (x ≤ 10) nanopowders. The lower valence states of Co2+ induces oxygen vacancies which transforms some Ce4+ to Ce3+. A careful study of oxygen vacancies, strain, bond length and related band structure changes, have been targeted in this study. The possibility of phonon participation in electronic transition has been discussed using Tauc plot. Ce3+ forms defect states, between valence and conduction bands. Lattice parameters decrease, but strain increases with substitution. © 2018, Springer-Verlag GmbH Germany, part of Springer Nature.