Origin of the β1 parameter describing the nephelauxetic effect in transition metal ions with spin-forbidden emissions

Abstract

Recently a new nephelauxetic parameter β1=(CC0)2+(BB0)2, where B, C (B0, C0) are the Racah parameters of the transition metal ions in a crystal (free state), was introduced to rationalize the variations in the energy of the 2E-4A2 and 1E-3A2 spin-forbidden emission transitions that occur when ions with the d3 and d8 electron configurations are introduced as optically active impurities in a lattice. A critical analysis of the vast experimental data that is available in the archival literature shows that the energies of the above-mentioned transitions are linear functions of the β1 parameter. In this work, we further confirm this empirical finding and provide a derivation of the observed linear dependence by considering the Tanabe-Sugano matrices for the corresponding electron configurations.