The structural groups of alkali silicate glasses determined from ²⁹Si MAS-NMR

Lithium, sodium and potassium silicate glasses containing 20-56 mol% alkali oxide were investigated by ²⁹Si nuclear magnetic resonance (MAS-NMR) spectroscopy. In the spectrum of each sample, at least two to four distinct peaks were identified. The distributions of SiO₄ structural units, Q_n, where n is the number of bridging oxygen atoms bound to other Si atoms, were determined as a function of composition. The equilibrium constants of the reactions, 2Q_n ⇌ Q_n-1 + Q_n+1 (n = 3, 2, 1), were determined. The reaction proceeds to the right direction as cationic power of alkali ion (Z/r) increases (Li⁺>Na⁺>K⁺) at the same alkali oxide concentration. The apparent equilibrium constants of the above reactions are discussed along with a proposed thermodynamic model. The ²⁹Si chemical shifts assigned to each structural unit increase linearly with alkali oxide contents. The slope of these lines decreases as the numbers of attached bridging oxygen (BO) atoms decrease. The average chemical shifts also increase linearly with an increase of alkali content. A close relationship between the average chemical shifts and the theoretical optical basicity was observed.