Optical basicity, interpreted by means of Ce 5d level spectroscopy in ionic crystals

The concept of optical basicity based on spectroscopy of Tl⁺, Pb²⁺, and Bi³⁺ energy levels in oxidic media like glasses and molten salts will be tested against information and theoretical knowledge available on Ce³⁺ in oxide ionic crystals. The redshift of the 4f → 5d transition of Ce³⁺ does not provide a basis for determining optical basicity. It will be shown that the proper basis is provided by the shift of the whole 5d configuration. Reasons why optical basicity can be based on the redshift of the ¹S₀ → ³P₁ transition in Tl⁺, Pb²⁺, and Bi³⁺ will be discussed. The theory on the centroid shift in Ce³⁺ provides new insights in the relation between optical basicity and physical properties like anion polarizability and refractive index. An alternative method to calculate optical basicity from the chemical composition of a compound will be proposed both for oxide and fluoride compounds.