Nuclear magnetic resonance properties of silica glass under pressure up to 2 Mbar: A view from ab initio calculations

Abstract The structures of silica (SiO 2 ) glasses at high pressure provide insight into the thermo‐mechanical properties of chemically complex amorphous oxides upon stress, compression, and indentation. While much of our knowledge on glass structures has been garnered from high‐resolution solid‐state nuclear magnetic resonance (NMR) by probing the evolution of nuclear spins of nuclides of interest in oxide glasses, the NMR spectrum of oxide glasses under pressure, particularly above ∼100 GPa has largely remained unexplored. In order to identify the key aspects of such spin behavior and unveil the elusive origins of glass densification, we present theoretical calculations of prototypical silica glass, which provide us with a quantitative window to reveal the relations between structural changes in silica glass and the nuclear spin interactions up to 200 GPa. The simulation results confirm that average O coordination numbers increase with increasing pressure from 2 at 1 atm to 3.2 at 200 GPa. Upon compression, at low‐pressure conditions, the Si–O distance tends to increase with decreasing Si–O–Si angle. The negative correlation between the Si–O bond length and the Si–O–Si bond angle becomes more prominent with increasing pressure. The 29 Si isotropic chemical shielding (δ cs ) for higher coordinated Si is larger, and the average 29 Si δ cs linearly increases with the Si coordination number. The 17 O δ cs and quadrupolar coupling constant (C q ) decrease with the average oxygen coordination number. The 17 O δ cs of high‐pressure configurations, such as [3] O and [4] O linearly decrease with the decreasing Si‐O distance, and the 17 O C q of [2] O tends to increase with the increasing Si–O–Si angle. These results confirm an increase in topological structural disorder with increasing pressure above 100 GPa. The current simulation results establish the correlations between the NMR parameters for Si and O in oxide glasses up to 200 GPa and the pressure‐driven structural evolution in coordination numbers, bond angles, lengths, and pressures. The current correlations, along with our earlier results for MgSiO 3 glass, constitute the guideline for the utility of NMR to trace the densification paths of amorphous oxides under extreme pressure and stress.