Temperature and pressure dependence of the elastic properties of RbAg4I5

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Abstract
An ultrasonic pulse superposition technique was used to measure the velocities of the three pure‐mode waves which propagate along [110] in a single crystal of rubidium tetra‐silver pentaiodide. The temperature was varied from 25 °C, through a phase transformation at −65 °C, and to a second phase transformation at −151 °C. There was a peak in the ultrasonic attenuation at the −65 °C transition for the longitudinal and one of the shear waves, and the attenuation and velocity of all of the waves were completely reversible upon cycling the temperature through this transition. The lower‐temperature phase was generally several percent softer elastically than the room‐temperature phase. Attempts to make measurements through the −151 °C transition were not successful. The pressure dependence of the wave velocities were determined at 25 °C. The adiabatic second‐order elastic constants and their temperature and pressure derivatives at 25 °C which were determined from these measurements are as follows. The values of Cij (1011 dyn/cm2), C−1ij (∂Cij/∂T) (10−4 °K−1), and ∂Cij/∂P for C11 are 1.648±0.002, −3.54±0.12, and +8.73±0.11, respectively; for C12, 0.934±0.002, −3.73±0.13, and +6.29±0.11, respectively; and for C44, 0.4892±0.0005, −4.17±0.05, and +0.884±0.015, respectively. The Debye temperature of 90 °K and the thermal expansion coefficient of 0.566×10−4 °K−1 calculated from these values agree very well with values determined by other means.