The microscopic transition process from high-density to low-density amorphous state of SnI₄

A molecular crystalline SnI₄undergoes pressure-induced solid-state amorphization via molecular dissociation to the high-density amorphous (HDA) state, which we call Am-I. In the present study, we examine the reverse transition process from Am-I to the low-density amorphous (LDA) state, called Am-II. We first measure the structure factor on decompression from 30 GPa down to 1.1 GPa at room temperature, usingin situangle-dispersive synchrotron x-ray measurement and a diamond anvil cell. We then estimate the density, which exhibits an abrupt change between 3.3 and 3.0 GPa, indicating the HDA(Am-I)-to-LDA(Am-II) transition. We use the density and the molecular configuration generated from a molecular dynamics simulation as input to a reverse Monte Carlo fit. The fit vividly visualizes gradual molecular reassociation between 18 and 14 GPa within the Am-I region. The Am-I state can thus be divided into two states: the high-pressure Am-I state containing isolated Sn atoms and the low-pressure Am-I state consisting of deformed molecules connected by metallic I₂bonds. In the latter state, the molecular shape becomesC₃-like just before the transition to Am-II, in which molecules recover the originalTsymmetry. This local symmetry change has been detected on the liquid-liquid transition of SnI₄, suggesting the strong coupling between the local symmetry and the global order parameter of density.