X-ray excitation fluorescence spectra were investigated for $M{\mathrm{F}}_2:\mathrm{E}\mathrm{u}$ $(M=\mathrm{Ca},$ Sr, and Ba) and their mixed-crystal systems, ${\mathrm{Ca}}_{1-x}{\mathrm{Sr}}_x{\mathrm{F}}_2$ and ${\mathrm{Sr}}_{1-x}{\mathrm{Ba}}_x{\mathrm{F}}_2$ with the same fluorite structure. The UV recombination fluorescence band of the ${\mathrm{V}}_K$ center associated with blue emission due to the $f-d$ transition of ${\mathrm{Eu}}^{2+}$ ions was observed with changing mixture ratios x at room temperature. Two sets of weak spectra due to $f-f$ transitions of ${\mathrm{Eu}}^{3+}$ ions also appeared in the 500–600-nm wavelength region. The peak wavelengths and the integrated intensities of the observed fluorescence were investigated as a function of the Eu concentration as well as the mixture ratio. For the blue emission of ${\mathrm{Eu}}^{2+},$ pulsed x-ray excitation resulted in shorter lifetimes (500–800 ns) than optical excitation, suggesting energy transfers between the excited states of ${\mathrm{V}}_K$ centers and ${\mathrm{Eu}}^{2+}.$ A kinematical fluorescence mechanism was proposed, taking into account the formation of a close pair of a hopping ${\mathrm{V}}_K$ center and an immobile ${\mathrm{Eu}}^{2+}$ ion followed by an energy transfer from the former to the latter. Based on the calculated fluorescence decay curves best fitted to the response curves by x-ray pulse excitation, the energy transfer rates from ${\mathrm{V}}_K$ centers to ${\mathrm{Eu}}^{2+}$ were estimated.

• Received 13 April 1998

DOI:https://doi.org/10.1103/PhysRevB.60.11984