Kinetic mechanisms of the pressure-driven phase transitions of AgI

Ab initio periodic DFT-GGA-LCAO calculations at the athermal limit have shown that AgI transforms from the zinc blende to the tetragonal antilitharge structure at 1.2 GPa, and then to rock salt at 1.6 GPa. A monoclinic Pm pathway is proposed for both reconstructive phase transitions, which leads to a "bifurcate" three-step mechanism. One step relates the antilitharge structure to a metastable Bmm2 orthorhombic phase (with Ag and I in five-fold coordination), and is followed by two alternative steps transforming the metastable structure into either zinc blende or rock salt. The enthalpy curve along the Pm pathway shows two bottleneck states bracketing the orthorhombic phase, with a predicted maximum activation enthalpy of 0.088 eV. Changes of the Ag crystal-chemical environment account for the mechanism. The kinetics of direct zinc blende/rock salt conversion is also considered, within an orthorhombic Imm2 pathway, and it is compared to the previous mechanism.