In this thesis, the optical absorption spectra of cobalt doped $MgIn_2S_4$, $CdIn_2S_4$, $HgIn_2S_4$, $AgIn_5S_8$, $CuIn_5S_8$ and β-$In_2S_3$ spinel crystals measured at 7 K were studied. The photoluminescence (PL) spectra of $MgIn_2S_4$ single crystals were also investigated as a function of temperature and excitation laser intensity. Single crystals were grown by a chemical transport reaction method using iodine as a transporting medium.
The PL band of $MgIn_2S_4$ was observed to be centered at 1.62 eV at 10 K and an excitation intensity of $0.02 Wcm$^{-2}$. The PL intensity decreased with increasing temperature and a rapid thermal quenching of the PL band was observed above 130 K. The red-shift of this band with increasing temperature was also observed. To explain the observed PL behavior, we propose that the emission is due to radiative recombination of a donor-acceptor pair, with an electron occupying a donor level located at 0.05 eV below the conduction band, and a hole occupying an acceptor level located at 0.62 eV above the valence band. Therefore, the blue-shift of the PL band with increasing excitation laser intensity is explained using the inhomogeneously spaced donor-acceptor pair model.
The optical absorption spectra of cobalt doped $MgIn_2S_4$, $CdIn_2S_4$, $HgIn_2S_4$, $AgIn_5S_8$, $CuIn_5S_8$ and β -$In_2S_3$ spinel crystals measured at 7 K showed several peaks due to the $Co^{2+}$ impurities which were well explained in terms of the $T_d$ crystal field followed by the second order spin-orbit coupling. For $CdIn_2S_4$:Co$^{2+}$, the additional fine structure caused by the small trigonal ($C_{3v}$) distortion at Cd sites and the vibrating replicas was observed in the $^{4}A_2(^{4}F)→$^{4}T_1(^{4}F)$ transition. We could not observe any absorption peaks of cobalt ions with octahedral symmetry for all these crystals. But because the absorption of cobalt ions in the octahedral symmetry could be too weak to be observed, the possibility of cobalt ions being present in the octahedral sites cannot be excluded.
The crystal field parameter Dq, the Racah parameter B, and the spin-orbit parameter λ obtained from the observed spectra show that the delocalization of the d electrons of $Co^{2+}$ ions is influenced by the Co-S bond length and the electronegativity difference between the metal (Me) and sulfur atoms of the host crystal. For cobalt doped spinel crystals, the $d$ electron delocalization increases in the order of $MgIn_2S_4$, $CdIn_2S_4$, $HgIn_2S_4$, $AgIn_5S_8$, $CuIn_5S_8$ and β -$In_2S_3$, and $CuIn_5S_8$. From this order, it can be concluded that the d electron delocalization increases with decreasing the Co-S bond length and the electronegativity difference of Me-S bond. The large $d$ electron delocalization in the β-$In_2S_3:Co^{2+}$ crystal means that vacancies at tetrahedral sites also increase the d electron delocalization.
