Analysis of new, promising wide-band-gap nonlinear crystals for the mid-IR is given. We focused on ternary and quaternary chalcogenides and based on their physical and chemical properties, which in addition to linear and nonlinear optical properties, determine the efficiency of crystal usage in certain laser experiments. The correlation between the composition, structure and output parameters as well as the trends in changes of the main characteristics such as band gap, nonlinear susceptibility, laser damage threshold, etc, are demonstrated. Vibrational spectra of Li-containing nonlinear crystals and their application in THz radiation generation are discussed. Using an example of LiGaS 2 , the defect effect on spectroscopic characteristics is shown. High-temperature annealing in a proper atmosphere allows one to remove extended and point defects and to improve the transparency. Energy structure and parameters of main point defects such as anion vacancy (F-center) and cation antisite defect Ga Li calculated from the first principles agree well with the experimental results.

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