Ultrasonic Velocity around the Successive Phase Transition Points of CsPbBr3

Sound velocities in perovskite-type crystal CsPbBr 3 have been measured around the structural phase transition points at 88 and 130°C. “Detwinned” samples were used, which made it possible to relate the measured velocities to the elastic constants in each phase. The velocities of c 11 and c 11 – c 12 modes exhibit large anomalies above and below the two transition points, whereas that of c 44 mode is almost independent of temperature throughout the transition region. The results are compared with the consequences of phenomenological and fluctuation theories. Temperature dependence of the velocity of c 11 mode just above the higher transition point is shown to be consistent with the prediction from the fluctuation theory,