FIELD: optical single crystals production. SUBSTANCE: invention relates to a technology for obtaining optical single crystals based on solid solutions of silver halides of the AgCl_0.25Br_0.75 - AgI system, intended for the design of devices in medical technologies, laser systems for a wide range of applications, thermography and THz vision devices, including security systems. The method includes melting a mixture based on salts of a single-phase solid solution with a purity of 99.9999 wt.% in a Pyrex glass ampoule in an installation that implements the vertical Bridgman method, and moving the ampoule to a zone with a low temperature, while melting a high-purity charge in the form of a dispersed solid solution of the AgCl_0.25Br_0.75 - AgI system is carried out at a temperature of 400-450°C with subsequent movement of the ampoule at a speed of 1-1.5 mm per hour to a zone with a temperature of 120-150°C, then annealing is carried out in the installation at 100°C for 20 hours to obtain single crystals based on the AgCl_0.25Br_0.75 solid solution, additionally containing silver iodide, in the following ratio, mol.%: AgCl_0.25Br_0.75 - 98.0-84.0; silver iodide - 2.0-16.0. EFFECT: obtained terahertz single crystals are photo- and radiation-resistant, non-toxic, non-hygroscopic, plastic, highly transparent from the visible to the far IR range (0.47-45.0 μm), and in the terahertz region from 0.1 to 30.0 THz are opaque in the range from 6.5 to 1.2 THz, which corresponds to the optical range from 45 to 250 µm. 1 cl, 1 dwg, 3 ex.