We have studied the photoluminescence (PL) of Ca x Ba 1-x Ga 2 S 4 solid solutions (x = 0.1, 0.2, 0.3, 0.4, and 0.5) activated by ions of the rare-earth element Eu 2+ in the temperature range 10-300 K, due to 4f 6 5d → 4f 7 transitions in the Eu 2+ ions. The photoluminescence spectra of Ca x Ba 1-x Ga 2 S 4 :Eu are located in the blue-green -green regions, and their maxima are shifted from 506 nm for x = 0.1 to 531 nm for x = 0.5. We show that the long-wavelength shift as Ca content increases is due to the increasing crystal fi eld, in the presence of which the 5d orbitals of the europium ions are split and consequently the energy of the recombination transitions decreases. We observed high temperature stability of the position of the photoluminescence spectra and the integrated intensity (decreasing by 13% for all x) in the temperature range 10-300 K. We have established that an increase in the Ca content in Ca x Ba 1-x Ga 2 S 4 :Eu leads to a decrease in the thermal shift of the photoluminescence, and for x = 0.5 there is no shift.Introduction. Synthesis of high-effi ciency luminophores emitting in the visible region of the spectrum is a key problem to be solved in designing devices for visualization and illumination that are competitive with conventional systems. Promising luminescent media include ternary chalcogenide semiconductors, in particular barium thiogallate (BaGa 2 S 4 ), strontium thiogallate (SrGa 2 S 4 ), and calcium thiogallate (CaGa 2 S 4 ) activated by rare-earth ions Eu or Ce [1-5]. These compounds differ from the binary compounds (GaS, Ga 2 S 3 ) in their resistance to hydrolysis and their good ability to incorporate rare-earth ions into their crystal lattice [5]. The compounds BaGa 2 S 4 and CaGa 2 S 4 activated by Eu 2+ ions display intense emission in the blue-green (~500 nm) and yellow (~555 nm) regions of the spectrum, due to 4f 6 5d → 4f 7 electronic transitions in the europium ions [1]. They are characterized by high thermal stability of the spectrum and photoluminescence (PL) intensity, and exhibit extreme stability of the position and shape of the spectrum and linearity of the photoluminescence intensity over a broad range of pulsed optical excitation all the way up to 10 4 W/cm 2 [5]. The presence of intense bands in the violet-blue region in the photoluminescence excitation (PLE) spectra of BaGa 2 S 4 :Eu and CaGa 2 S 4 :Eu makes them promising for effi cient excitation by the emission from commercial GaN and InGaN light-emitting diodes (LEDs) and laser diodes (LDs). Recently there have been studies of the optical properties of mixed compounds MGa 2 S 4 -M ' Ga 2 S 4 (M, M ' = Ba, Sr, Ca (alkaline earth elements) activated by rare-earth ions, in which variation of the composition of the matrix leads to a shift of the spectrum in the visible region [6,7]. Such mixed compounds are novel and their photoluminescence has not been suffi ciently studied.With the aim of varying the emission wavelength within the blue-green-yellow regions of the spectrum (505-555 nm), we synthesized novel...