Paramagnetic Resonance Spectrum of Trivalent Gadolinium in the Cubic Field of Calcium Fluoride

The paramagnetic resonance spectrum of Gd³⁺ in CaF₂ has been analyzed at 3 cm wavelength at room and liquid helium temperatures. The spectrum shows that the cubic symmetry is essentially preserved around the gadolinium ion. The fourth-order and sixth-order cubic field splitting parameter have been evaluated as c=+0.0185±0.0005 and d=-0.0004±0.0002 cm^-1, where the total ground-state splitting is 8c-2d. The g factor is g=1.991±0.002. Various mechanisms causing the ground-state splitting of an S state in a cubic field are discussed. The preservation of cubic symmetry and the absence of association with vacancies or interstitial ions can be explained by a stabilization of the complex by the spherical charge distribution of the S ion and an effective strong crystalline field.