Acousto-optic figures of merit of TlBr-TII solid solution (KRS-5) crystals are calculated for different propagation directions and polarizations of interacting waves. The transverse, collinear, and semicollinear regimes of acousto-optic interaction are considered in this paper. Optimal diffraction configurations applicable in acousto-optic devices operating in the far-infrared region of spectrum are found in the cases of longitudinal and shear acoustic waves in the cubic crystalline material. (C) 2017 Society of Photo-Optical Instrumentation Engineers (SPIE)