Reflection Spectra of Lanthanides in Thorium (IV) Oxide and the Large Nephelauxetic Effect of Oxide Ligands or Vacancies

Abstract

Retaining the cubic structure, ThO₂ can accomodate large concentrations of lanthanides and the equivalent amount of oxygen vacancies. The reflection spectra are measured at 100°K of Nd(III), Sm (III) , Eu (III) , Ho (III) , Er (III) , Tm(III) and Yb(III) in such samples. The excited J-levels correspond to rather broad bands in Thi_1-xM_jO_2-0.5x whereas Nd₂0₃ and Er₂0₃ have very detailed line groups. The visible spectra indicate a very large nephelauxetic effect in the oxide, the nephelauxetic ratio β being smaller than in the aqua ion to the extent of 0.033 for Nd(III), 0.019 for Sm (III). , 0.017 for Ho (III) , 0.016 for Er (III) and 0.013 for Tm (III) . These values are roughly five times larger than those for the anhydrous chlorides and are quite unexpected.