Structural model for amorphous aluminosilicates

An analytical model is developed for the composition-dependent structure of the amorphous aluminosilicate materials (M₂O)_x(Al₂O₃)_y(SiO₂)_1-x-y and (MO)_x(Al₂O₃)_y(SiO₂)_1-x-y, where 0 ≤ x ≤ 1 and 0 ≤ y ≤ 1. The model is based on a simple set of reactions and contains a single adjustable parameter p (0 ≤ p ≤ 1). The latter is found from ²⁷Al solid-state nuclear magnetic resonance (NMR) experiments in the regime where R = x/y ≥ 1, aided by new experiments on the magnesium and zinc aluminosilicate systems. The parameter p decreases linearly as the cation field strength of M⁺ or M²⁺ increases, as per the observation previously made for the degree of aluminum avoidance [Lee et al., J. Phys. Chem. C 120, 737 (2016)]. The results indicate that as the cation field strength increases, there are less fourfold coordinated aluminum atoms to contribute toward the glass network, and Al-O-Al bonds become more prevalent in a progressive breakdown of Loewenstein's aluminum avoidance rule. The model gives a good account of the composition-dependent fraction of non-bridging oxygen (NBO) atoms for R ≥ 1, as assessed from the results obtained from solid-state NMR experiments. An extension of the model to (M₂O₃)_x(Al₂O₃)_y(SiO₂)_1-x-y glasses leads, however, to an excess of NBO atoms, the proportion of which can be reduced by invoking network-forming fivefold coordinated Al atoms and/or oxygen triclusters. The model provides a benchmark for predicting the structure-related properties of aluminosilicate materials and a starting point for predicting the evolution in the structure of these materials under the extreme conditions encountered in the Earth's interior or in processes such as sharp-contact loading.