Effect of a Ce3+ Codopant on the Gd3+ EPR Spectrum of SrF2 at Room Temperature

The room temperature Gd³⁺ EPR spectrum in SrF₂ containing Ce³⁺ as a second dopant exhibits three types of symmetry sites: cubic, trigonal, and tetragonal. Relative intensities of the spectra from these sites have been determined as a function of the Ce³⁺ codopant level in crystals having nominally 0.1 molar % of Gd³⁺. At low Ce³⁺ concentrations (< 0.1 molar %), the tetragonal-to-trigonal intensity ratio is 1.85 ± 0.25, with the cubic site intensity accounting for less than one tenth of the total observed intensity. At higher Ce³⁺ concentrations (up to 0.8 molar %), the cubic site intensity grows and the intensity of both axial sites decreases. This increase of cubic intensity is at variance with the generally accepted model of nonlocalized cubic compensation of the Gd³⁺, and a phenomenological explanation based on a local cubic phase in which rare earth ions are essentially surrounded with interstitial compensating fluorines is presented.