For the development of fiber optics for the range from 0.2 to 50.0 μm, one needs light-stable, nonhygroscopic, ductile crystals that would be transparent within this spectral range and have a lack of cleavage, and from which the flexible infrared (IR) fibers are extruded. The crystals based on solid solutions of silver and monadic thallium halides meet the conditions listed above. Consequently, by differential thermal and x ray analyses, we study the TlBr–TlI phase diagram using the crystals with optimal compositions, which we grow ourselves. We also manufacture light-stable nanocrystalline IR fibers that are transparent at longer wavelengths compared with AgCl–AgBr fibers.