Simulation of pressure-driven phase transitions from tetrahedral crystal structures

Pressure-driven transitions of ionic materials from the zinc-blende to rocksalt and $\delta -{\mathrm{ZnCl}}_2$ to ${\mathrm{CdCl}}_2$ crystal structures are studied using constant-stress molecular dynamics with a polarizable-ion potential model. Both transformations are characterized by a change in cation coordination environment from tetrahedral to octahedral and are nonmartensitic. Transformation mechanisms are identified and characterized and similarities discussed. The blende to rocksalt transformation is observed to proceed via a diatomic $\beta$-tin-like structure, though this is shown to be a transition state and not a true intermediate phase in this system. The relationship of the observed mechanisms to those deduced from experiments on halide systems is discussed. The development of displacive motion across the simulation cell is discussed. The ${\mathrm{ZnCl}}_2$ system is a layered structure, and while the coordination changes are highly cooperative within each layer, the overall transformation takes place on a layer-by-layer basis. In the blende, the interlayer correlations required to produce a grain-boundary-free final structure are associated with a shearing motion which propagates across the cell. These differences have characteristic effects on the kinetics of the transformations.