Ab Initio Investigation On The Properties Of Hydrogen Peroxide Under High Pressure

With the rapid development of high pressure technology, it makes that the experiments on the determination of material structures have more reliable basis, using Raman signal and X-ray signals, we can indirectly learned the internal structure information of the material, thus we get a deep understanding of the phase transition under different conditions(different temperature, pressure). The structure determines the nature of the material, thus we know the corresponding properties under different conditions. Hydrogen peroxide is very common kind of material, and in daily life also has a wide application. At present the research work only stays in the experimental stage, but the theoretical work slims.The most significant experimental progress is the neutron diffraction test of the hydrogen peroxide by William R. Busing in the year of 1965. He modifies the previous XRD data, and determines the accurate atom position of hydrogen, thus the structure of phase I is finally confirmed. The experimental samples are prepared from 90% hydrogen peroxide by evaporation under vacuum at room temperature until about one-tenth of the starting material remained. It is later shown from observations of the freezing point that the concentration of H₂O₂ in this residue is greater than 99%. Finally each sample is frozen slowly to a single crystal, at this time the temperature of the samples decreases from -1℃to about 5℃.Hyunchae Cynn studies the phase transition and decomposition of the 90% hydrogen peroxide in 1999. The research shows that the hydrogen peroxide changes to phase I at room temperature under 1.5 GPa. Under 7.5 GPa phase I transforms to the high pressure phase (phase II). When reaches the melting point, hydrogen peroxide decomposes to water and oxygen. It is notable that the Raman spectroscopy collected in this experiment shows there are some extra peaks at 80K under ambient pressure, indicating the possible phase transition from phase I to phase I', but the structure of phase I'is still unknown. Hydrogen peroxide at high temperature under low pressure is not stable, and easily decomposes. But it is stable at room temperature under high pressure. H₂O₂-I is optically anisotropic, as expected from its tetrahedral crystal structure, and can easily be distinguished from isotropic ice-VII cubic crystals or fluid O₂ under cross polarizers. The XRD data shows that hydrogen peroxide transforms from phase I to phase II between 7 and 8 GPa. But in this interval, some X-ray diffraction peaks from phase I also exist, phase I and phase II are very likely to coexist at 11.5GPa. This shows that during the phase transition there is a strong dynamics barrier.In 2010, Jing-Yin Chen proves by experiment that: the tetragonal phase (H₂O₂-I) is stable to 15 GPa, above which transforms into an orthorhombic structure (H₂O₂-II) over a relatively large pressure range between 13 and 18 GPa. Above 18 GPa H₂O₂-II gradually decomposes to a mixture of H₂O and O₂, which completes at around 40GPa for pure and 45 GPa for the 9.5% H₂O₂. Upon pressure unloading, H₂O₂ also decomposes to H₂O and O₂ mixtures across the melts, occurring at 2.5 GPa for pure and 1.5 GPa for the 9.5% mixture. The most concentrated solution studied in the experiment is determined to be > 97.5 wt% H₂O₂ which we refer to―pure‖.And the measurement in this experiment is more accurate. The experimental data is currently the most reliable structure data of hydrogen peroxide.As is known to all, the structure and phase transition of water are very complex. In a very small pressure range, water has several phase transitions. It increases the difficulty for people to study the water. The molecular of hydrogen peroxide has one more hydrogen atom than the molecular of water. People try to get some useful enlightenments through the investigation on the high pressure phase transition of hydrogen peroxide so that we can better research the complex phase transition of water. But the structure of H₂O₂ at low temperature under low pressure is still unknown. The structure at higher pressure is not finalized either. At present only the structure of phase I is known to all. But many properties of phase I haven't been studied carefully through the theoretic calculation. Therefore, we calculate the stability range of phase I and use the CASTEP module of MS package to study the properties of phase I of H₂O₂. First of all, the dynamical and mechanical stabilities of the crystal structure (P4₁2₁2) for phase I are validated by our theoretical calculations. The calculations have confirmed that the structure (P4₁2₁2) is dynamically and mechanically stable in the pressure range of 4-37 GPa. However, it is dynamically unstable below 4 GPa or above 37 GPa. The most credible experiment have shown that the phase transition from phase I to phaseâ…¡occurs in the range of 13-18 GPa, which proves the phase transition is contributed by neither soft mode transition nor unstable elastic constants. Jing-Yin Chen finds in experiments that there arises a new peak at 13.9GPa in the XRD data.We obtain the XRD of phase I through theoretic calculation, combining with the theoretically calculated XRD ofε-O₂ andâ…§-H₂O, rule out the possibility that the new peak comes from theε-O₂ orâ…§-H₂O .The new peak can only come from the new phase. Thus we confirm the phase transition from phase I to phaseâ…¡occurs at 13GPa around. The weak peak at 7.5°observed in experiments mark the occurrence of hydrogen peroxide structure transformation. We calculate the relationship of the lattice parameters and volume of phase I versus pressure. With the results of the experiment, we obtain B₀ = 19.3±1.0GPa,B₀′= 4.0,V₀ =18.0±0.2cm³/mol between the pressure range from 5GPa to 30GPa. Finally we calculate the band structure of phase I, it reveals that there is no metallization signs of phase I.