Permanent densification behavior and ²⁹Si MAS NMR of SiO₂-K₂O-CaO-SrO glasses

In permanent densification of xSiO₂ · 0.4(100 - x)K₂O · 0.4(100 - x)CaO · 0.2(100 - x)SrO glasses (x = 41.2-66.7) in mol% by the 3 and 6 GPa high-pressure treatments a peculiar behavior was found in the glasses of x below 50 mol%. The skeleton structures composed of SiO₄ units in the glasses were investigated on the undensified and densified glasses by means of ²⁹Si magic angle spinning (MAS) NMR spectroscopy. From the peak deconvolution of MAS NMR spectra the Q_n fractions of SiO₄ structural-units, where n (n = 0-4) means the number of bridging oxygen, were examined. At first, the skeleton structures in the undensified glasses are clarified. Then the skeleton structures of the densified glasses are discussed in comparison with those of the undensified glasses. Irrespective of the x value, the fractions of Q_n of small n-values increased with an increase in the applied pressure, indicating that some of the Si-O-Si bonds are broken under the applied pressure and the skeleton structures were kept even after a release of the applied pressure. It is concluded that other causes should be considered as the origin of the present peculiar behavior.