Extent of Disorder in Magnesium Aluminosilicate Glasses: Insights from ²⁷Al and ¹⁷O NMR

Abstract

The quantification of the intrinsic disorder in archetypal noncrystalline magnesium aluminosilicates remains unsolved. This lack of knowledge is because of the increased structural perturbation caused by Mg2+, a high field strength cation, resulting in substantial broadening in both spectral and scattering responses. Most progress regarding amorphous aluminosilicate has thus been made with relatively low field strength cations (e.g., Na+ and Ca2+). Here, we quantified the nature of structural disorder in Mg-aluminosilicate glasses in the enstatite (MgSiO3)-pyrope (Mg3Al2Si3O12) join using 17O and 27Al NMR. While Mg-aluminosilicate glasses show a much larger topological and configurational disorder around Al than those of Na- and Ca-analogues, the fraction of [5,6]Al (∼8–10%) and the magnitude of topological disorder do not vary significantly with composition. This implies spatial proximity between Mg2+ and the under-bonded bridging oxygens, such as Al-O-Al and Si-O-Al, while Mg2+ preferentially forms Mg-O-Si over Mg-O-Al. The estimated degree of Al avoidance (Q) of ∼0.65 for Mg-aluminosilicates based on 17O NMR is close to a random distribution of Si/Al (Q = 0) and is thus much smaller than those estimated for Na- and Ca-aluminosilicate glasses (from ∼0.95 to ∼0.85) that often show evidence for Si/Al ordering (Q = 1, complete Al avoidance). The results also revealed that degree of Al avoidance decreases linearly with increasing cation field strength of non-network-forming cations, highlighting the first simple predictive relationship between the nature of chemical disorder and the types of non-network forming cation. This established correlation can be utilized to explain and predict the diverse properties of the Mg-bearing multicomponent glasses and melts with complex composition-dependence.

Effects of Difference in Ionic Radii on Chemical Ordering in Mixed‐Cation Silicate Glasses: Insights from Solid‐State 17O and 7Li NMR of Li–Ba Silicate Glasses

“…Figure shows the 17 O 3QMAS NMR spectra of the Li–Ba silicate glasses; the Li–O–Si, {Li, Ba}–O–Si, Ba–O–Si, and Si–O–Si peaks are better resolved. The peak assignments of the BOs and NBOs in the 17 O 3QMAS NMR spectra are partially based on previous studies on crystalline and amorphous end‐member Li and Ba silicates . In 17 O 3QMAS NMR spectrum for Li 2 Si 2 O 5 glass, the Li–O–Si (~−29 ppm in the isotropic dimension, and ~31 ppm in the MAS dimension) and Si–O–Si (~−46 ppm in the isotropic dimension) are completely resolved.…”

“…While the preferential proximity between the network‐modifying cations and the diverse oxygen species (e.g., BOs and NBOs) is suggested to be controlled by the differences in nature of non‐network cations, the effects of the difference in the ionic radii on chemical ordering in mixed‐cation silicate glasses remain to be fully understood. Although previous studies on Ba–Mg (~0.63 Å) and K–Mg (~0.66 Å) silicate glasses may address the effect of the ionic radii on the degree of cation mixing, the observed nonrandomness in these glasses is attributable to the distinct nature of Mg 2+ (i.e., strong affinity between Mg 2+ and NBOs), rather than the difference in ionic radii on mixing between these cations . Thus, in order to establish the effects of the difference in the ionic radii on chemical ordering, study of other mixed‐cation silicate glasses with a larger difference in ionic radii has been anticipated.…”

Cationic effect of charge compensation on the sulfide capacity of aluminosilicate slags

“…Q = 0 indicates that Si and Al are randomly distributed within the unit structure. According to previous studies, the degree of aluminum avoidance ( Q ) varies with cation species (Ca 2+ : ~0.875, Mg 2+ : ~0.65, Li + : ~0.9, and Na + : ~0.95). Experimental studies have shown that Q ranges from 0.8 to 0.875 in the metaluminous join ().…”

“…According to previous studies, 36,40 the degree of aluminum avoidance (Q) varies with cation species 40 (Ca 2+ :~0.875, Mg 2+ :~0.65, Li + :~0.9, and Na + :~0.95). Experimental studies 36,40 have shown that Q ranges from 0.8 to 0.875 in the metaluminous join (X CaO =X Al 2 O 3 ¼ 1). In this study, as the influence of composition on the range of Q was smaller than the cation effect, for convenience, the maximum value Q = 0.875 was employed in the model.…”

Amorphous Oxides under Extreme Compression: Insights from Solid-State Nuclear Magnetic Resonance and Inelastic X-ray Scattering

“…수행되었다. [13,14] 또한 Al-27 NMR 연구로부터 다성분계 비정 질 산화물 내의 구성양이온들의 세기(cation field strength)가 증가할수록 화학적 -위상학적 무질서도가 증가하고, 고배위수 Al( [5] Al, [6] Al)의 양이 증가하는 것을 정량 적으로 확인하였다. Al, [5] Al, [6] Al)이 관찰된다.…”