We have experimentally studied acoustic wave (AW) velocities in Tl 3 AsSe 3 crystals. Basing on these results, the matrices of elastic stiffness and compliance coefficients are obtained and the cross sections of the AW velocity surfaces by the principal crystallographic planes are constructed. We have revealed a phenomenon termed as an acoustic isotropic point. We have found that, at the room temperature and the AW frequency equal to 10 MHz, the Tl 3 AsSe 3 crystals are close to the conditions

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... f acoustic isotropy, which are analogous to the isotropic point known in crystal optics. Namely, the cross sections of the AW velocity surfaces under these conditions are almost circular and the difference between the velocities of transverse AWs with mutually orthogonal polarizations is very small. Then the specific relations among the elastic stiffness tensor components hold true, which are peculiar for the isotropic glass materials: C 11  C 33 , C 12  C 13 , C 44  C 66  (C 11 -C 12 )/2, and C 14  0. We have revealed that the Tl 3 AsSe 3 crystals exhibit very small obliquity of the acoustic energy flow with respect to the wave vector of the AW, as well as negligibly small angles of deviation of acoustic polarization from the purely transverse or longitudinal types. The acoustooptic figures of merit calculated for the interaction of waves propagating close to the principal crystallographic axes are as high as 3068×10 -15 and 3262×10 -15 s 3 /kg, respectively for the isotropic and anisotropic types of acoustooptic interactions.