Photoluminescence of $Ce^{3+}$ ions in $Ca_{1-x}Sr_xS$ solid solutions

Luminescence spectra of $Ce^{3+}$ in $Ca_{1-x}Sr_xS$ ($0 < x <1$) have been measured following excitations in the activator's band and at the fundamental absorption edge. It is shown that the model of a crystal field of cubic symmetry can be used to explain, in general, the spectrum evolution with changing concentration upon activator's excitation (with the exception of $x$ = 0.05 and 0.9), but this model cannot account for the peculiarities of the spectra arising upon excitation at the fundamental absorption edge. It is suggested that with band-to-band excitation a radiation-induced distortion of part of cerium luminescence centers occurs. A step-like variation of the luminescence parameters was detected as $x$ changed from 0 to 1, which seems to indicate that solid solutions are locally inhomogeneous.