Elastic properties of silver iodide at high pressure and temperatures: Thermally activated velocity anomaly in the B1 phase

Sound velocities have been measured in polycrystalline silver iodide at pressures !up to 1 $\text{GN}\text{m}{}^2$ and temperatures up to 425 K. The measurements covered the P-Tranges of three polymorphs of AgI but the main phase of interest was the B1 phase stable at high pressures. The B1 phase exhibits a large positive pressure derivative and a large negative temperature derivative of longitudinal velocity, both derivatives decreasing considerably at high pressures and low temperatures. The anomalous velocity behavior can be described by considering it due to a thermal activation process with ΔH¹ ≈ 0.4 eV and ΔV¹ ≈11 $\text{cm}{}^3\text{mole}$. The known importance of Frenkel defects in B4 AgI, and cation disorder and anomalously low sound velocities in B23 AgI suggest that Frenkel defects may be important in producing the velocity anomaly. The elastic properties of the different phases of silver iodide show rather complex relationships between lattice defects, cation disorder and sound velocities.