Search for precursor of pressure-induced amorphization of molecular crystal SnI4: Thermodynamic stability of low-pressure crystalline phase

Abstract

The pressure-temperature phase diagram of SnI 4 was investigated to examine the inherent instability of crystalline SnI 4 in terms of undergoing pressure-induced solid state amorphization, by conducting molecular dynamics simulations prior to studies involving laboratory experiments. The SnI 4 molecules are regarded as rigid tetrahedra interacting with one another via van der Waals forces. In order for the isothermal-isobaric ensemble to be achieved, the well-established Nosé -Klein scheme combined with the momentum scaling method was adopted when carrying out the simulations. The system was carefully heated up under fixed hydrostatic pressure from the low-pressure crystalline state across the melting point, which was determined by monitoring the time-dependence of the mean square displacement of the molecules, whereas, on cooling, the liquid state remained supercooled down to room temperature. The slope of the liquidus in the phase diagram between the low-pressure crystalline phase and the liquid phase was found to be positive, implying that the low-pressure crystalline phase has little connection with solid state amorphization. An expected overall pressure-temperature phase diagram of SnI 4 is discussed.

Polyamorphism in tin tetraiodide

“…Moreover, the samples that melted below the break pressure could recover the Pa3 structure at ambient conditions, whereas those melted above that pressure could not. 16 This, plus the finding that the initial rapid increase in the melting point with pressure can be explained by the crystalline model consisting of rigid molecules interacting with van der Waals forces, 17 implies some qualitative change in the molecular form. 18 With this background, we conducted synchrotron x-ray diffraction measurements for the liquids just above their melting points, marked by the blue and pink circles in the pressure-temperature phase diagram shown in Fig.…”

Amorphization and decomposition of scandium molybdate at high pressure

“…1,2 A wide variety of compounds with different types of bonding such as covalent ͑SiO 2 ͒, ionic ͑LiKSO 4 ͒, molecular ͑SnI 4 ͒, and hydrogen bonding ͑H 2 O͒ have exhibited the phenomenon. [3][4][5][6] In addition, systems with more complex bonding such as those in silicates, 7 tungstates, 8 molybdates, 9 and other minerals 10 are also found to undergo amorphization at high pressure. A number of factors such as metastable melting, 6 kinetic hindrance of phase transition, 11 dynamical instability, 12 polytetrahedral packing, 13 and orientational disorder of polyatomic ions 14 have been argued to be responsible for this.…”

Effect of octupole–octupole interactions on the thermodynamic stability of the low-pressure crystalline state of SnI4 molecular crystals

“…As a result, the melting points predicted from molecular dynamics (MD) simulations [2] were found to be too high. That is, the crystalline model adopted by the MD simulations was excessively stable.…”

“…The wellknown 12-6 form [3] is adopted for the LJ potential f LJ . The parameters 3 and s denote the fundamental energy and the length scales, respectively, [2]. The central tin atom is electrically shielded by the iodine atoms and does not explicitly contribute to the interaction.…”