Raman Spectroscopy Of Hydrogen Deuterium And Hydrogen Argon Mixtures At Low Temperature And High Pressure

In physics,pressure is an essential thermodynamic parameter.Many substances exhibit exotic properties under extreme pressures,and some unconventional new substances are discovered.High-pressure physics,the study of changes in the properties and structure of matter under extremely high-pressure conditions,provides a new direction for humanity to explore the universe.How to transform hydrogen,the simplest element in the universe,into a dense metal has become one of the most important problems in high-pressure physics.In 1935,Wigner and Huntington first tried to predict hydrogen would take on a metallic state as pressure increased.With the increase of extreme pressure and the development of ultra-high pressure experimental techniques,hydrogen has been successfully compressed at 400 GPa.The experimental techniques of in situ Raman and infrared spectroscopy of solid hydrogen at ultra-high pressure have been extended to a broader range of pressures and temperatures.Loubeyre,a leading French expert in high-pressure physics,found significant spectroscopic evidence for the transition of solid hydrogen to the metallic phase through in situ infrared absorption spectroscopy measurements.Electrical studies by Eremets et al.at the Max Planck Institute in Germany showed that above 360 GPa,solid hydrogen started to conduct electrically,with electrical properties similar to semimetals.So far,no consensus on the discovery of metallic hydrogen,and experimental evidence is still lacking.The available theoretical results indicated that a pressure of 500 GPa was required to obtain metallic hydrogen.The crystal structures of the high pressure and low temperature phases of hydrogen are currently controversial,and there are still difficulties in reducing the hydrogen metallization pressure.Therefore,in this paper,we investigated the effects on the structure,phase diagram and properties of solid hydrogen by introducing other molecules.We chose to study two typical gas molecules that form mixtures with hydrogen:one was a common isotope of hydrogen,deuterium（D）,which had twice the relative atomic mass of hydrogen and the same valence electrons.The other was a rare gas,argon（Ar）,with a full shell layer valence electron arrangement.In situ high pressure Raman spectroscopy was performed at low temperature for samples with a volume ratio of 1:1 hydrogen deuterium mixtures and 1:1 and 9:1 hydrogen argon mixtures,and the following conclusions were obtained.（1）For the hydrogen deuterium mixture with a volume ratio of 1:1,the phase boundary of the transition from phase I to phase II at pressure up to 153 GPa was modified in the temperature range of 80 K-300 K.In situ high pressure and low temperature Raman spectroscopy observed the asymmetric broadening of three vibrational modes in the mixture.The rate of change of the vibrational mode HD and D₂ was significantly higher than H₂ in the mixture with increasing pressure at the same temperature.These experimental phenomena indicated that deuterium atoms were more strongly coupled in the mixture environment and support theoretical calculations of the localized,mass disorder of the hydrogen deuterium mixture in the internal environment.（2）For the hydrogen argon mixture with a volume ratio of 1:1,we observed that the vibration mode（υ₁）of Ar（H₂）₂ was less affected by temperature with the changing trend of pressure in the range of 58-185 GPa.Below 190 K,new vibration mode（υ₀）appeared near 4526 cm^-1.The appearance and disappearance of this mode was directly related to temperature.A new low temperature phase of Ar（H₂）₂ and the stable range of its existence were determined.For the hydrogen argon mixture with a volume ratio of9:1,compressed along the two isotherms of 80 K and 110 K.It was found that the trend of hydrogen vibrational mode with pressure in the hydrogen argon mixture was similar to that of pure hydrogen.However,the Raman frequency shift of the hydrogen vibrational mode showed a significant hysteresis phenomenon,which was considered as an obstacle to the hydrogen metallization process.This hindering behavior was more important at a high ratio of 1:1 argon doping.