AgGa_1-xIn_xS₂ and AgGa_1-xIn_xSe₂ nonlinear crystals with variable chemical composition are novel perspective materials for parametric conversion of the laser radiation. With the In quantity increase the principal refractive indices rise and the value of birefringence changes. The possibility of the controlling In quantity alteration makes it possible to produce the crystals with optimum phase matching angles for the given parametric process, specifically to realize the non-critical processes . The shown curves indicate that for different compositions the minimum phase matching angle practically corresponds to the same pumping wavelength and rises with In quantity increase. The same behavior is typical and for AgGa_1-xIn_xSe₂ crystals. To this moment on AgGa_1-xIn_xSe₂ elements (x = 0.3) the SHG of CO₂ laser with phase matching angle near 90^oand process efficiency equal 60% was realized. The main properties of AgGa_1-xIn_xS₂ and AgGa_1-xIn_xSe₂ crystals are investigating. The high-performance frequency doublers of CO₂ laser radiation (range 9.2-10.6 mm) are designed on the basis of AgGa_1-xIn_xSe₂ crystals. The power conversion efficiency is about 50-60%.