Elasticity through Nanoscale Distortions in Periodic Surfactant-Templated Porous Silica under High Pressure

High-pressure infrared absorption spectroscopy is used to examine changes in local bonding upon hydrostatic compression in both ordered surfactant-templated mesoporous silica and sintered sol−gel silica with a goal of connecting atomic scale structural changes with variations in nanoscale periodicity. High-pressure IR absorption spectra are analyzed on the basis of a noncentral force model. It is found that the intertetrahedral bond angle and its distribution width in both the dense and the mesoporous silica decrease at elevated pressure up to 4 GPa. With increasing pressure above this value, decreases in the average bond angle and distribution width cease in the mesoporous silica, while they continue in the bulk material. The results suggest that in the mesoporous silica the nanometer length scale of the silica framework makes it energetically unfavorable to form high-density atomic scale structures at higher pressures (>4 GPa). Instead, further compression of the mesoporous silica takes place by distortion of the periodic pore structures on the nanometer scale (deformation of pores). Surprisingly, upon the release of pressure, structural changes on both the nanometer and atomic length scales are reversible. The results suggest that reversible nanometer scale distortions in periodic porous materials can replace the irreversible atomic scale distortions observed in bulk amorphous silica.