Structure and related properties of amorphous magnesium aluminosilicates

The structure of the magnesium aluminosilicate glasses (MgO)_x⁢(Al₂⁢O₃)_y⁢(SiO₂)_1–x–y, where 0 ≤ x < 1, 0 ≤ y < 1 and x + y < 1, was explored by neutron and x-ray diffraction, aided by the results from ²⁷Al magic angle spinning nuclear magnetic resonance spectroscopy. A wide composition range was investigated, using aerodynamic levitation with laser heating to extend the glass-forming region well into the peraluminous regime where R = x/y < 1. The results were interpreted with the aid of an analytical model for the composition-dependent structure where magnesium ions do not contribute towards the glass network. The model delivers Al-O bond distances typical of fourfold and fivefold coordinated aluminum atoms. It also delivers Mg-O coordination numbers of 4.46(9) and 5.39(10) for magnesium in a predominantly network-modifying versus charge-compensating role, corresponding to Mg-O bond distances of 2.024(10) and 2.120(20) Å, respectively. The more compact coordination environment of the network modifier is related to an enhanced probability of magnesium finding nonbridging oxygen (NBO) atoms as the nearest neighbors. Structural change is observed along the tectosilicate tie line in the transformation from a “normal” to “anomalous” glass, where the mechanism of deformation under sharp-contact loading changes from shear flow to densification. A minimum in the composition dependence of the glass hardness is related to a minimum in the Al-O coordination number and to a competition between the availability of NBO atoms, which break the connectivity of the tetrahedral network, versus high cation field-strength Mg²⁺ and fivefold and sixfold coordinated Al³⁺ ions, which cross-link the pieces of the network thus fragmented.