The optical absorption spectra of 1% Gd:LaBr3, 5% Gd:LaCl3, GdCl3, 2% Gd:Y(OH)3 and Gd(OH)3 at 1.5 K in high resolution are given over the region 31 000 cm-1 to 41 000 cm-1. Reduced matrix elements based on intermediate-coupling free-ion calculations are used to perform a crystal-field analysis for Gd3+ in a 6¯(C3h) symmetry electrostatic field. A least-squares fit to levels in each crystal has led to the assignment of M quantum numbers to most levels and of Bkq parameters to each crystalline host.
R. L. SCHWIESOW and H. M. CROSSWHITE, "Energy Levels of Gd3+ in Five Hexagonal Crystals," J. Opt. Soc. Am. 59, 592-601 (1969)