Probing the high-pressure densification of amorphous silica nanomaterials using SBA-15: An investigation into the paradoxical nature of the first sharp diffraction peak

The densification and X-ray scattering of mesoporous silica (SBA-15) were measured simultaneously under gigapascal (GPa) pressures. The results are compared to previous work on amorphous silica (aSiO₂) and demonstrate the feasibility of measuring the densification of aSiO₂ nanomaterials with small angle X-ray scattering (SAXS) in-situ in a diamond anvil cell. Compared to fused silica, the position of the SBA-15 first sharp diffraction peak (FSDP) is 7 times more sensitive to pressure and has a transition in its pressure dependance at a lower pressure (∼2 GPa vs. ∼13 GPa). SBA-15 has two densification regimes, low-density amorphous and high-density amorphous, which have equations of state comparable to low-density amorphous and high-density amorphous fused silica. The transition between these two regimes occurs at a lower pressure than for fused silica (∼1.5 GPa vs. ∼13 GPa). The results suggest that there is no direct relationship between the FSDP position and the aSiO₂ density during compression.