Study of Yb$^{3+}$ optical centers in fluoride solid solution crystals CaF$_{2}$–SrF$_{2}$–YbF$_{3}$

A concentration series of Ca$_{(1-X)}$Sr$_{X}$F$_{2}$ : 4 mol % YbF$^{3+}$ crystals with $X$ varying from 0 to 1 has been grown. The absorption spectra of Yb$^{3+}$ ions in these crystals are measured at temperatures of 300 and 77 K. It is shown that the intensity ratio of the absorption lines of clusters and tetragonal centers in the congruent crystals is higher than that in the other CaF$_{2}$–SrF$_{2}$ solid solution crystals. Analysis of the absorption profiles of Yb$^{3+}$ optical centers in the concentration series of the solid solution crystals shows that it is energetically favorable for the Sr$^{2+}$ ion to enter the local environment of both the tetragonal center and the cluster.