Compressibility of vitreous silica by high pressure X-ray microtomography

Early work to determine the compressibility of vitreous silica at high pressure (< 5 GPa) was performed by linear displacement measurements (Bridgman, 1939). From these data Bridgman concluded that vitreous silica becomes more compressible with increasing pressure up to approximately 3 GPa. More recent experimental work using strain gauge techniques also indicated similar anomalous compressibility between 2-3 GPa (refs. 1, 2). Molecular dynamic simulations suggest two different explanations for this anomalous behavior - polyamorphism and existence of a flexibility window (refs. 3, 4). Support for the latter interpretation is provided by recent Raman spectroscopy (ref. 5) and MD simulations (ref. 6) suggesting that as vitreous silica is initially compressed (below 5 GPa) larger tetrahedral rings form that then can more easily fold back onto themselves, while at high pressures this distorted network structure become more inflexible. We used volume rendering of X-ray microtomography data obtained at the 13-BM-D beamline at the Advanced Photon Source (Argonne National Laboratory) to determine directly the changes in the volume of vitreous silica during compression to 8 GPa. The compressibility was determined for two different vitreous silica samples: Corning high-purity fused silica (HPFS™) containing ~1000 ppm OH and Corning waveguide silica prepared by outside vapor deposition with a hydroxyl content of ~ <10 ppb. In both cases, there is no evidence for anomalous compressibility in the pressure range of 2-3 GPa. The compressibility of HPFS silica is well-described using the Birch-Murnaghan (B-M) equation of state with a bulk modulus (Ko) of 15 GPa and K’ = 6. Waveguide silica exhibits more complex volume changes with pressure requiring Ko ≥ 25 GPa and significant stiffing at pressures > 3 GPa. This behavior cannot be modeled using the B-M EOS. The higher compressibility and more normal behavior of HPFS silica compared to waveguide silica may be related to differences in hydroxyl content and thus the abundance of weaker Si-O-H bonds. 1. El’kin et al., 2002, JETP Lett., 75; 2. Tsiok et al, 1998, Phys. Rev. Lett., 80; 3. Trachenko and Dove, 2003, Phys. Rev. B, 67; 4. Walker et al., 2007, J. Phys.: Condens. Matter, 19 ;5. Deschamps et al., 2009, J. Non-Cryst. Solids, 355; 6.Huang and Keiffer, 2004, Phys. Rev. B, 69.