First-Principles Studies Of Phonon Instabilities In Agi Under High Pressure

The dynamical instabilities of AgI are extensively studied using the density-functional perturbation theory to probe the mechanism of the pressure-induced phase transitions of wurtzite (WZ)/zinc-blende (ZB). tetragonal. rocksalt (RS). monoclinic. orthorhombic. CsCl. Analysis of the phonon calculations suggested that the pressure-induced instabilities of the transverse acoustic (TA) modes at the M/X points of zone boundaries for WZ/ZB AgI are responsible for the phase transitions of WZ/ZB. tetragonal. While the tetragonal. RS transition is not induced by phonon instability, but probably by energetic instability. Moreover, it is found that the transformation of the RS to monoclinic phase is driven by the softening of the TA mode at point X. The elastic constant calculations suggest that although the predicted elastic instabilities in WZ and ZB cannot independently induce the phase transition, they might couple with the softening phonon to lower the transition pressure. Pressure-induced metallization has been predicted for CsCl-type AgI from evidence of a significant band gap overlap. However, the CsCl phase of AgI is predicted to be dynamically unstable at the pressures under study.