X-ray excitation luminescence of the Eu ion doped into fluorite-type CaF2-SrF2 and SrF2-BaF2 mixed crystal systems

Luminescence spectra by X-ray excitation have been investigated for Eu2+ doped fluorite-type Ca1-xSrxF2 and Sr1-xBaxF2 mixed crystal or solid solution systems. While changing the mixture ratio x (0 less than or equal to x less than or equal to 1), mainly two kinds of spectra were followed; the UV band from electron trapping by V-k centers in the host fluorite crystals, and the blue band due to the transition from 4f(7) to 4f(6)5d of the doped Eu2+ ion. In the longer wavelength region, two weak bands were found, regarded as the luminescence originating from the Eu3+ ion. The peak wavelength, integrated intensity and lifetime of observed luminescence were investigated as the functions of the mixture ratio as well as the Eu concentration. A luminescence mechanism was proposed based on the energy transfer taking into account lattice distortions due to creation of mixed crystals, a hole migration from Eu2+ Eu3+ through V-k center and the degeneracy of excited states of Eu2+ in BaF2.