To obtain non-toxic, non-hygroscopic materials used in IR fiber optics and optoelectronics, a comprehensive study of the AgCl0.25Br0.75 – AgI system was carried out. The study of the AgCl0.25Br0.75 – AgI system's phase diagram showed the presence of a homogeneity region with a content 0–16 mol.% AgI in AgCl0.25Br0.75 and heterogeneity region 16–98 mol.% AgI in AgCl0.25Br0.75. The phases in the homogeneous region have a cubic structure of the Fm3m type. The phases in the heterogeneous region are a mixture of cubic Fm3m and hexagonal P63mc phases. Within each of the regions, single-crystals samples with compositions of 4, 8, 16, and 20 mol.% AgI in AgCl0.25Br0.75 and optical ceramics ones of composition 20 and 33 mol.% AgI in AgCl0.25Br0.75 were synthesized. For a sample of 20 mol.% AgI in AgCl0.25Br0.75, different synthesis modes were used, as a result it was possible to obtain both a single-crystal and ceramics. The materials' band gap and the optical transmission range are determined. The band gap decreases from 2.5195 to 2.3724 with a rise in the fraction of AgI in AgCl0.25Br0.75. The materials are transparent in the range of 0.492 - 54 μm. Single-crystals are characterized by a stepwise increase in transmission up to 50–57% at the short-wavelength absorption edge. For optical ceramics, the transmission initially reaches 1% stepwise and then increases until reaching a maximum value in the IR of 15–20 μm. It is associated with scattering by grains of different phases. A uniform shift of the transmission spectrum to longer wavelengths from 492 to 525 nm in the visible and from 47 to 54 μm in the far IR with a rise in the proportion of AgI in AgCl0.25Br0.75 is shown. The single-crystals' and optical ceramics' highest transmission level reaches 75–78% in the wavelength range from 10 to 40 µm.