Optical absorption ofZnGa2S4andZnGa2S4:Co2+crystals

Polycrystals of ${\mathrm{ZnGa}}_{2}$${\mathrm{S}}_{4}$ and ${\mathrm{ZnGa}}_{2}$${\mathrm{S}}_{4}$:${\mathrm{Co}}^{2+}$ were prepared from high-purity elements at 1270 \ifmmode^\circ\else\textdegree\fi{}C. The optical-absorption spectra of these polycrystalline powders were measured in the wavelength region 300--3000 nm with use of an integrating sphere and photoacoustic spectrophotometer at 298 K. The optical energy gap is found to be 3.18 eV for the ${\mathrm{ZnGa}}_{2}$${\mathrm{S}}_{4}$ crystal and 2.60 eV for the ${\mathrm{ZnGa}}_{2}$${\mathrm{S}}_{4}$:${\mathrm{Co}}^{2+}$ crystal. The optical-absorption peaks in the measurement of ${\mathrm{ZnGa}}_{2}$${\mathrm{S}}_{4}$:${\mathrm{Co}}^{2+}$ crystal absorption spectrum at 3790, 6049, 13 725, 20 704, and 22 779 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ are found to be the electron transition of the ${\mathrm{Co}}^{2+}$ ion with ${\mathit{T}}_{\mathit{d}}$ symmetry from the ground state $^{4}$${\mathit{A}}_{2}$${(}^{4}$F) to the excited states $^{4}$${\mathit{T}}_{2}$${(}^{4}$F), $^{4}$${\mathit{T}}_{1}$${(}^{4}$F), $^{4}$${\mathit{T}}_{1}$${(}^{4}$P), $^{2}$${\mathit{T}}_{2}$${(}^{2}$G), and $^{2}$E${(}^{2}$G). The crystal-field parameter and the Racah parameter, which were obtained from the optical-absorption peaks, are found to be Dq=379 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and B=560 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, respectively. The $^{4}$${\mathit{T}}_{1}$${(}^{4}$P) state of the ${\mathrm{Co}}^{2+}$ ion splits into three state of ${\mathrm{\ensuremath{\Gamma}}}_{6}$, ${\mathrm{\ensuremath{\Gamma}}}_{8}$, and ${\mathrm{\ensuremath{\Gamma}}}_{8}$+${\mathrm{\ensuremath{\Gamma}}}_{7}$ by first-order spin-orbit coupling effects at 298 K. The value of the spin-orbit coupling parameter was \ensuremath{\lambda}=-436 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$.