Raman Evidences of Chemical Reaction of NaCl-O₂ System at High Pressure and High Temperature

In this study we studied the chemical reaction of the NaCl-O₂ system under high pressure and high temperature using the diamond anvil cell apparatus and the double-sided laser heating technology.The sample was heated to about 1 500-2 000 K under a pressure of 55 GPa, and then analyzed by the Raman measurement at room temperature.The determined data show that the NaCl-O₂ system undergoes a chemical reaction under the above-mentioned pressure and temperature conditions.The products include the nontraditional compounds of NaO₄ and NaCl₃, as well as a small amount of NaClO₄ and intermediate product Cl₂.The black NaO₄ powder, being apt to absorb moisture, can still exist in the ambient condition.The diagnostic zero-pressure Raman band of 1 384 cm^-1, assigned to the symmetric stretching vibration of the O₄^- anion in NaO₄, suggests the occurrence of O-O atom pair with a negative fractional valence in the structure.The orthorhombic structure of NaCl₃ exhibited a strong Raman signal, which could distinguish 10 Raman peaks in our measurements.It decomposes into NaCl+Cl₂ under about 23 GPa on decompression.The experimental results suggest that both elements of oxygen and chlorine prefer to occur in the unconventional form of pair-anions or polyanions caused by their unique chemical reactivity under high pressure, differing from that in atmospheric and lower pressure environments.It requires more robust evidences from various experimental systems to confirm whether these properties of oxygen and chlorine under high pressure are universal.Oxygen is one of the most important constituent elements on Earth, and most of the Earth is in high temperature and high pressure environment.This study provides new evidence for the possible unconventional existence of oxygen in the mantle.