Raman Evidences of Chemical Reaction of Re-H₂O System at High Pressure and High Temperature

Rhenium tablet is a frequently used gasket material at the ultra-high pressures in Diamond anvil cell (DAC) experiment. Water in deep Earth is the link between material exchange and energy circulation in the Earth’s interior. It is greatly scientific and technical significance on the study of chemical reaction of Re-H₂O system at high pressures and temperatures. Microscopic observations and Raman measurements show that the Re-H₂O system takes place the redox reaction ${2{{\rm{H}}_2}{\rm{O}} + {\rm{Re}}\;\;\;\begin{matrix}{40.5\;{\rm{GPa}}} \\\hline \hline{1\;800\;{\rm{K}}}\\\end{matrix}\;\;\;{\rm{Re}}{{\rm{O}}_2} + 4{\rm{H}}}$ under the conditions of high pressures and high temperatures, and produce rhenium oxide (β-ReO₂) with Re⁴⁺ and atomic hydrogen (H). Observed fourteen characteristic Raman peaks of oxidation product ReO₂ have a continuous unequal shift to lower Raman frequencies with the release of pressure. Reduction product H does not further take place interreaction with the water molecules, rhenium metals and their reaction products β-ReO₂ and atomic H under high pressures. But the hydrogen molecules are formed when the pressure is released to near atmospheric pressure. The chemical reaction of Re-H₂O system under the conditions of high-pressure and temperature reveals that water (hydroxyl) can decompose to produce atomic hydrogen in the Earth’s interior with the high pressure, high temperature and reductive material. This discovery not only provides a new experimental evidence for the conversion of water to hydrogen in deep Earth, but also gives important basis for exploring the possible geochemical behaviors of water in the Earth’s interior.