The structure of the mixed fluorites Ca1-xSrxF2 and Sr1-xBaxF2, as well as the structure of the Eu2+ impurity center in these crystals, is calculated within the framework of the virtual-crystal method realized in the shell model and pair-potential approximation. The phenomenological dependence of the position of the lower level of the 4f65d configuration of the Eu2+ ion on distance to the Eu2+-ligand is derived. The dependences of the Stokes shift and the Huang-Rhys factor on x are calculated for the yellow luminescence in Sr1-xBaxF2:Eu2+. The value of x at which the lower level of the 4f65d configuration of the Eu2+ ion in Sr1-xBaxF2:Eu2+ falls within the conduction band is found.