Exotic structural motifs in aluminosilicate glasses quantified by solid-state NMR and molecular dynamics simulations

Abstract

Conventional structural models of aluminosilicate (AS) glasses assume that there are no direct bonds between four-coordinated Al (Al[4]) species and non-bridging oxygen (NBO) anions, along with the (near) absence of Al[4]–O–Al[4] linkages (the “Loewenstein Al avoidance rule”). Yet, accumulating evidence from advanced solid-state NMR experiments on AS glasses that incorporate (moderately) high field-strength cations, notably so those of rare-earth metals (RE3+), reveal significant Al[4]–NBO and Al[4]–O–Al[4] populations. Here we review such NMR experimentation that enable a direct probing of these “exotic” structural motifs. We also re-analyze and discuss previously presented experimental and modeling results of a large set of RE2O3–Al2O3–SiO2 glasses with RE={La, Y, Lu, Sc}, where the structure/composition-related parameters that dictate the degrees of Al[4]–NBO and Al[4]–O–Al[4] bonding in AS glasses are identified, leading to new quantitative composition–structure relationships. Moreover, for a given AS composition, criteria are presented for whenever Al[4]–O–Al[4] linkages must exit in its glass structure.