Hydrogen solubility in cristobalite at high pressure.

Area of Science:

• Materials Science
• High-Pressure Physics
• Solid-State Chemistry

Background:

• Cristobalite is a high-temperature polymorph of silica (SiO2).
• Understanding hydrogen incorporation in silica polymorphs is crucial for geological and materials science applications.

Purpose of the Study:

• To investigate the loading of hydrogen into cristobalite-I under high pressure.
• To characterize the state and location of hydrogen within the cristobalite structure after high-pressure treatment.

Main Methods:

• Powder samples of cristobalite-I were subjected to hydrogen loading at pressures up to 90 kbar and 250 °C.
• Samples were quenched under pressure to liquid nitrogen temperature.
• Characterization involved X-ray diffraction, Raman spectroscopy, and thermal desorption analysis at ambient pressure.

Main Results:

• Hydrogen content increased with pressure, reaching a H2/SiO2 molecular ratio of approximately 0.10 at 90 kbar.
• Quenched samples contained ~80% cristobalite-I and ~20% cristobalite-II-like phases with expanded crystal lattices.
• Raman spectroscopy indicated that hydrogen is dissolved as H2 molecules within the cristobalite phases.

Conclusions:

• High pressures facilitate significant molecular hydrogen incorporation into the cristobalite silica structure.
• The presence of hydrogen leads to lattice expansion and a phase transformation towards a cristobalite-II-like structure.