Impurity Luminescence in ${\rm BaF} _{2}$:${\rm Y} ^{3+}$ and ${\rm BaF} _{2}$:${\rm Yb}^{3+}$ Crystals

This paper presents both investigations of high-energy luminescence with maximum at 7.5 eV in BaF
₂
:Y
³⁺
and BaF
₂
:Yb
³⁺
crystals and theoretical calculations of cross-luminescence spectrum in crystals of barium fluoride with interstitial ions of fluorine. The dependencies of the 7.5 eV emission intensity as function of different impurities mol. concentration are presented. The intensity of this emission increases with concentration of rare-earth impurity, but in the case of doping by Yb
³⁺
and Y
³⁺
this intensity is less than that in the case of La
³⁺
doping. This feature (as well as the nonlinear dependence) can be due to the creation of clusters, i.e., aggregation of impurity ions and F
^-
-interstitials. The intensity of the 7.5 eV luminescence in a BaF
₂
:Y
³⁺
crystal after annealing at 1000 K increased by three times but it remained less than in the case of La
³⁺
doping. Theoretical calculations of the core hole electronic and spatial structure and cross-luminescence spectrum were performed in the [Ba
₆
F
₁₂
] cluster. This cluster contains interstitial ion of fluorine, which is surrounded by six ions of Ba
²⁺
. In the case of using Hartree-Fock method the calculated luminescence spectrum of BaF
₂
crystal with interstitial fluorine shows cross-luminescence band with main maximum at 5.2 eV and high-energy band at 11.7 eV. In the case of using TD DFT method the energies of both cross-luminescence transitions and of transition between interstitial ion and core levels are in satisfactory agreement with experimental data, being all overestimated by approximately the average value of 0.8 eV.