Structural mechanisms of compression and decompression in high-pressure K2Si4O9 glasses: an investigation utilizing Raman and NMR spectroscopy of glasses and crystalline materials

This study uses Raman, 29Si magic-angle spinning nuclear magnetic resonance (MAS NMR) and 17O triple quantum (3Q) MAS NMR spectroscopies on K2Si4O9 glasses quenched from high pressure melts (5.7 and 8 GPa) and on high-pressure crystalline phases of K2Si4O9-wadeite and CaSi2O5 to investigate the structural mechanisms that create high coordinated silicon. The effect of decompression on the glass structure was also investigated by varying the decompression rates after temperature quench. The spectrum of crystalline triclinic CaSi2O5 phase clearly demonstrates that the [4]Si–O–[5]Si species do not show a distinct signature in 17O 3QMAS NMR and their signal can either be represented as “[4]Si–O–[4]Si-like” or “[4]Si–O–[6]Si-like” species, depending on the local environment of the oxygen. This suggests that Si-coordination should be directly investigated by 29Si NMR and not inferred from 17O NMR spectra. Additionally, based on the comparison of percentages of structural species measured with 17O NMR to those expected from the 29Si spectra, it seems that most, if not all, [4]Si–O–[5]Si in rapidly decompressed K2Si4O9 glasses are represented as [4]Si–O–[4]Si, not [4]Si–O–[6]Si. These results were used to successfully test previously proposed mechanisms for the generation of high-coordinated Si (Q3+Q4=[5]Si+Q4 and 2Q3+Q4=[6]Si+2Q4) when NBO are present. Spectra from glasses that were decompressed more slowly (conventionally) show that there are small structural differences between the glasses with different rates of decompression. Based on 29Si and 17O data, a small percentage of [6]Si converts to [5]Si (∼1% of the total Si) by the reversal of the previously mentioned mechanism. Interestingly, this small structural change shows a relatively large effect on the Raman spectra, which suggests that the effect of decompression on silicate glass structure may need re-evaluation with additional in-situ studies.