Divalent rare-earth ions Pr, Sm, Нo, Er, Tm, and Yb in crystals of alkaline-earth fluorides
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The absorption spectra of radiation-colored CaF2, SrF2, and BaF2 crystals activated by trivalent Pr, Sm, Нo, Er, Tm, and Yb (rare-earth, RE) ions are studied. It is shown that ionizing radiation reduces the impurity ions to the divalent state. The temperature resistance of divalent RE ions of radiation-colored CaF2 crystals correlates with the chemical stability of the compounds with divalent RE ions. The photochromic centers are produced in CaF2-Pr crystal colored by radiation at room temperature and heated to 200°C.
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