Many crystals with chalcopyrite structure possess outstanding nonlinear optical properties. A common feature of these compounds is the presence of scattering centers in as-grown crystals due to decay of the solid solutions. While the basic principles of how to suppress the centers are known, this contribution is aimed to detailed microscopic study of these defects on the example of AgGaS₂ (AGS) crystals. Due to high sulfur pressure at the melting point of AGS (996˚C) a separate synthesis of raw material is necessary. In this work the synthesis of AGS was performed by direct fusion of the elementary compounds in the single-zone furnace. Then, the charge was placed in the double-wall ampoule for recrystallization by modified Bridgman method.  Thin plates of AGS from as-grown crystal as well as from that annealed in Ag₂S and Ga₂S₃ atmosphere were taken for study. The annealing was done by heating the sample and the boat with agent in evacuated quartz ampoules. Various times and temperatures were tested. No difference was found between the samples by X-ray diffraction and Raman measurements. However, according to EDS X-ray microprobe analysis, the scattering centers consist from inclusions of Ag₂Ga₂₀S₃₁ phase. Elimination of these inclusions takes place after annealing of the samples in the atmosphere of Ag₂S. Diffusion of Ag-rich components into the bulk of AGS determines the time of annealing. Around 2 weeks are necessary for boundary between milky and optically clear AGS to propagate at 1 cm inside the sample at 850˚C. The composition of inclusions is also confirmed by the fact that annealing with Ga₂S₃ makes the inclusions larger which in turn results in cleavage micro cracks in the sample. A certain attention has been paid to acid etching of the samples. The treatment in HCl revealed morphological feature of the plate like inclusions which explains why they look like a dot-lines in transmitted light. Also this acid effectively dissolves defective outer layer on the polished surface of the AGS, while the undamaged cleavage plane {112} remains unchanged. On the other hand HNO₃ reacts with any surface of AGS, so this acid is suitable to reveal etch pits.

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