Elastic stiffness, acoustic absorption, and photoelastic constants of the crystal have been determined at room temperature using ultrasonic light diffraction and pulse-echo methods. Two acoustic modes have been found to be particularly useful to acousto-optical applications: One is the shear mode propagated in the [110] direction with displacement along [110], which is characterized by a remarkably high acousto-optical figure of merit, M (n 6p2/ρv3) = 793×10-18 sec3/g; and the other is the longitudinal mode along [001] with high figure of merit defined taking account of frequency bandwidth, M′ (n 7p2/ρv) = 142×10-7 cm 2·sec/g, and low acoustic loss. © 1969 The American Institute of Physics.