The optical transitions due to the cobalt transition metal impurity centers with composition x in C d1-k Co x Ga 2 S 4 mixed crystals were investigated by optical absorption spectroscopy. The optical absorption spectra showed three crystal-field transitions between the ground state 4 A 2 ( 4 F) and the excited states 4 T 2 ( 4 F), 4 T 1 ( 4 F), and 4 T 1 ( 4 P) of substitutional Co 2+ ions that were similar for all the investigated compositions. The crystal-field parameter (Dq) and the Racah parameter (B) obtained from the observed absorption bands showed a composition dependence such that the Dq parameter increases; on the other hand, the B parameter decreases with increasing composition x, which implies that the Racah parameter (B) depends strongly on the host crystal. Also, the total splitting between the highest and the lowest states of the 4 T 1 ( 4 P) state split due to spin -orbit coupling showed a composition dependence varying from 1576 cm -1 to 1232 cm -1 with increasing x. The spin -orbit coupling parameter λ varied from 394 cm -1 to 308 cm -1 with composition x.

PACS 78.55.HxPhotoluminescence spectra of Cd 1−xMnxGa2S4 mixed crystals were investigated in the composition region of 0.00 ≤ x ≤ 1.00 and in the temperature region of 6 to 300 K. The photoluminescence spectrum of CdGa 2 S 4 (x = 0.00) showed two emission bands at 594 nm (2.09 eV) and 460 nm (2.70 eV), which were attributed to be due to donor-acceptor pair recombination. The same feature of these bands was also observed for the crystals doped with 5 mol% Mn, excepting a slightly red shift and a decreasing of the halfwidth of the emission band in the longer wavelength region. For the crystals in the composition region of x ≤ 0.67, only the emission band in the longer wavelength region were observed. The photoluminescence spectra of MnGa 2S4(x = 1.00) showed a broad emission band 675 nm (1.838 eV), which is different from that of CdGa 2S4. The relative intensity of the main emission band increased in the composition region of 0.00 ≤ x ≤ 0.67 with increasing x while decreased in the region of 0.90 ≤ x ≤ 1.00. The anomalous composition dependence was connected with an occurrence of a miscibility gap of the crystal structure in that region.