The complete elastic constants tensor of monoclinic potassium cobalticyanide [K3Co(CN)6] has been determined from measured ultrasonic phase velocities. These were obtained as a function of direction in several crystal planes by measuring the acoustic energy reflection coefficient at a liquid–solid interface as a function of angle and plane of incidence. The measurements were made at 27.6 MHz in p-xylene at 21.5 °C. The elastic constants were determined using an existing perturbation method. Velocities calculated from the derived elastic constants are in agreement with the measured velocities to within experimental error and the approximations in the perturbation calculation. The usefulness of this method for the determination of the elastic constants of low symmetry crystals is thus established. The Debye temperature of K3Co(CN)6 is estimated from the results to be 290 ± 10 °K with a corresponding mean acoustic velocity of 2560 ± 80 m/s.
