The luminescence spectra which were excited by X-rays were investigated in Eu2+-doped fluorite-type mixed crystals or solid solutions. As the Sr or Ba content was increased, 2 types of spectra were monitored. These were the ultra-violet band which arose from electron trapping by Vk centers in the host fluorite crystals, and the blue band which was due to transitions, from 4f7 to 4f65d, of the doped Eu2+ ion. In the longer wavelength region, 2 weak bands were found and were regarded as being due to a luminescence that originated from the Eu3+ ion. A luminescence mechanism was proposed which was based upon an energy transfer that took account of lattice distortions that were due to the creation of mixed crystals, to hole migration from Eu2+ to Eu3+ via Vk centers and to the degeneracy of the excited states of Eu2+ in BaF2.

K.Kawano, R.Nakata: Materials Science Forum, 1997, 239-241, 761-4