Unified view for pressure-induced successive changes from amorphization to re-entrant crystallization

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A new idea of "thermodynamic optimization for spatial configuration among elements with variable size" is proposed. On the basis of this idea the reporter has tried to explain a series of structural changes induced by uploading pressure. To this end, a molecular crystal tin tetraiodide was chosen as a prototype substance, which undergoes pressure-induced amorphization followed by re-entrant crystallization. If tin atoms are assumed to occupy the tetrahedral interstitial sites of the host lattice made of iodines, frustration will come up in finding possible occupation sites as far as the tin does always take four iodines as nearest neighbors. According to this rule the lower-pressure phase structure automatically appears as a case without any frustration. However, frustration develops with increasing tin population. The local energy of the frustrated site becomes higher as compared to nonfrustated site. The Hamiltonian can then be written down by suitably assigning the local energies. T … More he series of structural variations were thus qualitatively explained by minimizing the free energy obtained through the Hamiltonian.The idea can be adapted to describe the physical origin of polyamorphism. It is naturally inferred that a pressure-induced liquid-to-liquid transition will take place in tin tetraiodide. In fact, from the melting curve of the low-pressure crystalline phase was found to have a break at around 1.5 GPa, beyond which the melting curve becomes almost flat. This suggests that the density of the low-pressure liquid differs from that on the higher-pressure side. Finally, we have succeeded in showing direct evidence for the existence of the two liquids with different local structures by conducting in situ synchrotron x-ray diffraction experiments. The present idea yields, when treated within mean-field approximation, a regular solution treatment. The phase diagram of tin tetraiode was constructed within this framework, which included a new critical point and spinodal lines. Less