Electron magnetic resonance of gadolinium-doped calcium fluoride

Electron magnetic resonance (EMR) spectra of gadolinium-doped calcium fluoride have been studied at room temperature for Gd concentrations between 0.01 and 2.00 mol%. Gd3+ ions in sites with two different symmetries were observed. One of the sites, with cubic symmetry, is unstable at room temperature and decays with a time constant of 2.2 day−1. The other site, with tetragonal symmetry, is stable and is attributed to Gd3+ ions in substitutional sites next to a charge-compensating F− interstitial ion. The linewidth and intensity of the EMR spectrum with tetragonal symmetry increase with increasing Gd concentration. A theoretical calculation based on the concentration dependence of the EMR linewidth yields an effective range of the exchange interaction between Gd3+ ions in CaF2 of 0.774 nm, of the same order as that of Gd3+ ions in other cubic ionic compounds.