In this thesis, the optical properties of $AgGaS_2$:$Co^{2+}$($T_d$ point symmetry),
$CuInS_2$:$Co$^{2+}$($D_{2d}$ point symmetry), and $AgInS_2$:$Co^{2+}$($C_1$ point symmetry) and the substitutional sites of $Co^{2+}$ ions in their single crystals were studied. They were synthesized by melt growth technique and grown by chemical transport reaction using iodine as a transporting medium.
At first, the energy level splittings of $Co^{2+}_d$ electrons in the $T_d$, $D_{2d}$, and $C_1$ point symmetry along with the spin-orbit coupling interaction were derived from the crystal field theory considering group theoretical calculation, which uses character tables for point-symmetry groups with spin-orbit coupling effect, and Kramers' theorem.
In $AgGaS_2$, the lattice constants of Co-doped crystals were almost the same as those of undoped one. The absorption spectra shows two groups of peaks at $5000 ~9000cm^{-1}$ and $12000~17000cm^{-1}$. Each has three or four peaks at 10 K. The analysis of the absorption spectra shows that the $Co^{2+}$ ions are located at the Ga sites($T_d$ symmetry), not the Ag sites ($D_{2d}$ symmetry). From the energy of transitions of $Co^{2+}$ in $AgGaS_2$, the crystal field parameter Dq, Racah parameter B, and spin-orbit constants $λ(^4T_1(^4 F))$ and $λ(^4T_1(^{4} P))$ were obtained to be $359 cm^{-1}$,
$605 cm^{-1}$; and $-188 cm^{-1}$ and $-115 cm^{-1}$, respectively. The similarity to the values of ZnS implies that the Co impurities replace the Ga atoms. In Raman spectra all the Raman modes were observed to soften in the Co-doped crystals, which also implies that the $Co^{2+}$ occupy the Ga sites, because the electronegativity difference of the Co-S bond is smaller than that of the Ga-S bond while larger than that of the Ag-S bond.
The lattice constants and the Raman spectra of the $CuInS_2$:$Co^{2+}$ single crystals were almost the same as those of undoped $CuInS_2$ single crystals. The optical spectra of $CuInS_2$:$Co^{2+}$ single crystals at room temperature showed two broad peaks and the higher energy peak had three sub-peaks. These peaks can be explained in terms of the electronic transitions of $Co^{2+}$ ions in $T_d$ symmetry along with spin-orbit coupling effect. The crystal field parameter Dq, the Racah parameter B and the spin-orbit constant $λ(^{4} T_{1} (^{4} F))$ were found to be $400 cm^{-1}$, $540 cm^{-1}$, and $-195 cm^{-1}$, respectively. The optical spectra of the $CuInS_2$:$Co^{2+}$ single crystals at 4.7 K in the $5000~9000 cm^{-1}$ region showed twelve peaks and these peaks can be analyzed using additional considerations of $D_{2d}$ symmetry and zero phonon replicas. This result shows that the tetragonal distortion of $T_d$ symmetry in $CuInS_2$ single crystals is small. The optical band gap energies of the $CuInS_2$:$Co^{2+}$ single crystals decreased as the doped $Co^{2+}$ ions increased.
The $AgInS_2$ compound is a unique ternary compounds which has an orthorhombic wurtzite-like phase with space group $Pna2_1 (C_{2v}^9)$. In orthorhombic $AgInS_2$, the symmetry of anion atoms around each cation is lowered from $T_d$ to $C_1$. We observed three groups of absorption peaks due to Co impurities in the regions of $12000~14000 cm^{-1}(700 - 840 nm)$, $4300~6300 cm^{-1}(1600 - 2300 nm)$, and $3100~3600 cm^{-1}(2800~3200 nm)$ at 6 K. The highest energy absorption peaks correspond to the transition $^4A_2(^4 F) \rightarrow ^4T_2(^4 P)$, the middle ones to the transition $^4 A_2(^4F)$→$^4T_2(^4F)$, and the lowest one to the transition $^4A_2(^4 F)$→$^4T_1(^4F)$ of $Co^{2+}$ ions in $T_d$ symmetry. The spin-orbit coupling constants $λ(^{4}T_{2}(^{4}P))$ and $λ(^{4}T_2(^4 F))$ is found to be about $-255 cm^{-1}$ and about $-169 cm^{-1}$, respectively. The absorption spectra were well explained in terms of the Td crystal field followed by the 2nd-order spin-orbit coupling, but not by $C_1$ crystal field. In fact, $AgInS_2$ with the chalcopyrite structure has an anion displacement 0.25, which means that S atoms remains at the original sites of the zinc-blende structure. This may also be true of orthorhombic $AgInS_2$, since locally both have the same structure. In addition, the lattice constant ratios $b/a$ and $c/a$ were larger than those of the ideal one by only 1.56% and 2.51% respectively. These two facts make our conclusion reasonable. The crystal field parameter and the Racah parameter are 300 and $590 cm^{-1}$, respectively, which are very close to those of $Co^{2+}$ in the binary analog, CdS. This may imply that Co atoms substitute In atoms, because Cd has the same electronegativity as In and the Cd-S bond length is very similar to those of In-S.
