  Piezooptic coefficients of Sn2P2S6 crystals are experimentally determined for
l=623.8 nm and T=293 K with the aid of interferometric technique. The
components of the elastic stiffness tensor for these crystals are calculated on
the basis of studies for the acoustic wave velocities. It is shown that
acoustooptic figure of merit can achieve extremely high values for Sn2P2S6
crystals (M2 - 2x10-12s3/kg2).
