Elastic and piezoelectric contants of silver-iodide: Study of a material at the covalent-ionic phase transition

We have studied the elastic and piezoelectric properties of AgI in the wurtzite phase. Transformed elastic constants for the zinc-blende phase are obtained and are compared with those of other covalently bonded compounds near Phillip's critical ionicity ($f=0.785$). This confirms the earlier reported trend in the approach to zero of the (C₁₁-C₁₂)2 elastic constant as the covalent to ionic phase transition is approached, indicating a lattice instability owing to a vanishing of the bending force constant. The room-temperature hydrostatic pressure dependence of the elastic constants exhibits a peculiar hysteretic behavior, possibly related to a sluggish conversion from wurtzite to zinc-blende structure. The temperature dependence of the elastic constants is also reported. The piezoelectric constants were determined.