Kinetic mechanisms of the pressure-driven phase transitions of AgI

Abstract

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 $1.2\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, and then to rock salt at $1.6\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. 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 $0.088\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. 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.