EPR study of clusters of rare-earth ions in mixed fluoride crystals

The EPR spectra of the (BaF₂)_{1 ? x} (CeF₃) _x system are studied for the concentrations x = 0, 0.001, 0.002, 0.005, 0.01, and 0.02. The appearance of new tetragonal centers is detected beginning from x = 0.002, the intensity of these centers being maximal at x = 0.01. The (CaF₂)_{1 ? x ? y} (CeF₃) _x (YF₃) _y double solutions with x = 0.001 and y from 0 to 0.02 are also studied. In addition to the ordinary tetragonal center, beginning from y = 0.001, a new tetragonal center appears with the same structure as in the previously studied mixed crystals based on BaF₂—namely, the Ce³⁺-□-R³⁺ chain elongated along the fourfold axis substitutes the Ca²⁺-Ca²⁺-Ca²⁺ and Ba²⁺-Ba²⁺-Ba²⁺ chains in regular CaF₂ and BaF₂ crystals (□ is the cation vacancy, and R³⁺ is the Ce³⁺, La³⁺, or Y³⁺ trivalent ion).