Densification of sodium and magnesium aluminosilicate glasses at ambient temperature: structural investigations by solid-state nuclear magnetic resonance and molecular dynamics simulations

Sodium and magnesium aluminosilicate glasses with compositions 20Na2O-20Al2O3-60SiO2 (NAS) and 20MgO-20Al2O3-60SiO2 (MAS) were subjected to a 12 and 25 GPa compression and decompression at room temperature, resulting in density increases from 3.7% to 5.3% (NAS) and from 8.2 to 8.4% (MAS), respectively. The pressurization at 25 GPa was done on 17O-enriched glasses, to facilitate characterization by 17O NMR. The structural changes associated with this process have been investigated by solid state 29Si, 27Al, 23Na, 25Mg, and 17O magic-angle spinning NMR and compared with the situation in thermally relaxed glasses and/or glasses prepared at ambient pressure. While in the Na aluminosilicate glass only subtle structural changes were observed in a sample densified at 12 GPa, the average coordination number of Al < CN(Al)> increases moderately from 4.00 to 4.26 by pressurization at 25 GPa. In the Mg-based system, < CN(Al)> increases from 4.34 to 4.57 to 4.83 in the sequence 10-4 GPa -> 12 GPa -> 25 GPa. The experimental result at 25 GPa was qualitatively confirmed by molecular dynamics (MD) simulations. Overall, pressurization results in more positive 29Si and 17O chemical shifts, most likely reflecting a reduction in the Si-O-Si and Si-O-Al bonding angles in the pressurized glasses. Furthermore, the results are also consistent with either an increased number of non-bridging O-atoms upon pressurization, or a larger number of Si-O-Al or Al-O-Al linkages. The significantly higher sensitivity of MAS, compared to NAS glass, to an increase in < CN(Al)> upon pressurization, provides a good structural rationale for its significantly higher crack initiation resistance. Superposition of 17O triple-quantum magic-angle spinning NMR spectra of 20MgO-20Al2O3-60SiO2 (MAS**) and 20Na2O-20Al2O3-60SiO2 (NAS**) glasses prepared at ambient pressure (AP) and pressurized and decompressed at 25 GPa at room temperature (HP(25)).