Analysis of the Cubic Crystal-Field Parameters for Gd3+ in the Alkaline Earth Fluorides

The optical spectra of the alkaline-earth fluorides doped with Ge3+ (MeF2:Gd3+, where Me represents Ca, Sr, or Ba) at octahedral sites are analyzed to determine the effect of lattice size on the cubic crystal-field parameters. The point-charge model predicts a linear dependence of the first-order crystal-field splitting on B04 which in turn varies inversely with the Gd3+–F− separation to the fifth power (R−5). The slopes of plots of log b40 vs logR yield values of − 2.1, − 2.7, and 0 for the S7 / 28, P7 / 26, and P5 / 26 multiplets, respectively. This apparent discrepancy is resolved by the inclusion of crystal-field interactions in all orders between the S8 and 6(P, I, D) multiplets. The log(A40〈r4〉) vs logR plot is found to be − 1.8 which agrees with the EPR result to within experimental error. The disagreement with the point-charge model is examined.