Correlations between ¹⁷O NMR parameters and local structure around oxygen in high-pressure silicates: implications for the structure of silicate melts at high pressure

In order to understand the correlations between ¹⁷O NMR parameters and the local structure around O in high-pressure silicates, ¹⁷O MAS and static NMR spectra on several crystalline alkali silicate and silica phases, as well as on ambient pressure glasses have been obtained. The results show that ¹⁷O NMR is capable of distinguishing nonbridging O atoms (NBO), bridging atoms (BO: ^[4]Si-O-^[4]Si), O atoms in the ^[4]Si-O-^[6]Si linkage, and O atoms coordinated to three ^[6]Si (^[3]O). NBO are characterized by small quadrupolar coupling constants (e²qQ/h, ~2.3 MHz), but their isotropic chemical shifts (δ_i) are very sensitive to the type of network-modifying cations. ¹⁷O NMR has also been applied to study the structure of alkali silicate glasses quenched from melts at high pressure. -from Authors