Ultraviolet spectra of Tl+, Pb2+ and Bi3+ probe ions in molten fluoride media

Abstract

Although the orbital-expansion (nephelauxetic) parameter, h, for fluoride is usually assigned a value of 0.8, frequencies of the ¹S₀→³P₁ transition obtained from the ultraviolet absorption spectra of Tl⁺, Pb²⁺ and Bi³⁺ probe ions in molten LiF–NaF–KF (46.5:11.5:42) indicate values of 1.38, 1.33 and 1.09, respectively. These are consistent with the absorption spectra of thin films of the four halides of each of the metal ions. It is shown that Tl⁺, Pb²⁺ and Bi³⁺ are capable of detecting the reduction in electron-donor power of fluoride when it is modified by polarising cations such as boron(III) or phosphorus(V) as in molten alkali-metal tetrafluoroborate or hexafluorophosphate. Addition of KF to molten alkali-metal tetra-fluoroborate results in a gradual replacement of the polarised fluorides by F^– ions in the coordination sphere of the probe ion rather than the direct formation of the discrete fluoro-complex of Tl⁺, Pb²⁺ or Bi³⁺; substitution is virtually complete at KF contents < 20%.