Chinese scientists and their cooperators explored phase transition in dense SO2 and reported their new discovery that they found the pressure-induced amorphization in dense SO2 and a reversible pressure-induced structural transformation between two amorphous forms of SO2: molecular amorphous and polymeric amorphous.
This work was done by a research team at Institute of Solid State Physics (ISSP), Hefei Institutes of Physical Science and was published in PNAS earlier this week.
SO2 is a simple molecule and plays a significant part in research on chemistry, physics of the Earth and atmosphere. While properties of similar solid molecular systems such as CO2 or N2 at high pressures have been extensively studied, more research on dense SO2 still needs to be done especially on its behavior and properties, which remains largely unknown. In this study, scientists took a closer look at this simple molecule through a combined experimental and computational effort which tried to describe some new and unexpected phenomena.
By using experimental techniques of Raman spectroscopy and X-ray diffraction at high pressures, they compressed sulfur dioxide up to 60 GPa with a diamond anvil cell and explored the phase transitions and structures of SO2 up to 60 GPa and at temperatures of 77-300 K. At 77 K and below 16 GPa, sulfur dioxide is crystalline. Compressed to 16 GPa, the sulfur dioxide in the crystalline phase undergoes pressure-induced amorphization and enters the amorphous phase of the molecular state. Further compressed to above 26 GPa, a phase transition occurs from the molecular amorphous phase (two coordinated sulfur) to the chain polymeric amorphous phase (three coordinated sulfur). Several different temperature paths have been done, and it is found that the phase transition path in dense SO2 is from crystalline to molecular amorphous phase and then to polymeric amorphous phase in the entire temperature range of 77-300 K, and phase transition path is reversible. Also, the amorphization pressure is different with temperature, about at 10-16 GPa at 77-300 K temperature range. Furthermore, to test this new observation, the team used the molecular dynamics simulations and the same phenomenon was also observed. And especially the high-pressure polymeric amorphous form was found to consist mainly of disordered polymeric chains made of 3-coordinated sulfur atoms connected via oxygen atoms, and few residual intact molecules.
Some substances are known to exist in several different structurally disordered solid states, which is called polyamorphism. The first and perhaps most celebrated example of polyamorphic behavior was discovered in water ice in 1984 by Mishima et al. Two different forms of amorphous water ice were identified, known as low density amorphous (LDA) and high-density amorphous (HDA) ices. Later on, similar phenomena have also been observed in other important systems such as Si, SiO2, and GeO2. In condensed matter physics, polyamorphism is a very interesting phenomenon. However, frankly speaking, this phenomenon remains poorly understood and needs to be investigated deeply and systematically. Here the Chinese scientists and their cooperators present this time a new example of such behavior in a simple molecular substance, sulfur dioxide, which was easily accessible experimentally since it occurs at moderate pressure. Moreover, the amorphous molecular to amorphous polymeric transition may suggest a possible existence of a similar transition in liquid state.
This work is supported by the National Nature Science Foundation of China, CAS President¡¯s International Fellowship Initiative, Science Challenge Project, CAS Innovation Fund and the Director¡¯s Fund of Science Island.
Figure 1. Vibrational spectra of solid SO2. (A and B) Selected Raman spectra of an SO2 sample measured upon (A) increasing pressure at 77 K and (B) decreasing pressure at room temperature. (C) Evolution of VDOS from ab initio MD simulations along compression (solid lines) and decompression (dashed lines) at T =300 K. Color represents structural state of the system: black, molecular crystal; red, molecular amorphous; blue, polymeric amorphous. (Image by LIU Xiaodi)
Figure 2. (A¨CC) Snapshots of the amorphous sample from MD simulations at different pressures. (A) The beginning of compression at 10 GPa where the sample consists only of SO2 molecules. (B) Structure during compression at 40 GPa where the sample contains both molecules and polymeric chains. (C) Structure after decompression to 10 GPa where the sample reverted from polymeric back to molecular. Simulation supercells are not to scale. (D and E) Comparison of polymeric chains in crystalline and amorphous forms of SO2. (D) Chains in crystalline Ama2 and Pmc21 phases. (E) Selected disordered chain in a configuration of a-SO2 from MD simulation at 30 GPa. The color of S atoms represents coordination: two-coordinated atoms (molecule) are yellow, and three-coordinated ones (polymeric chains) are blue-gray. (Image by LIU Xiaodi)