Local structures and Al/Si ordering in lanthanum aluminosilicate glasses explored by advanced 27Al NMR experiments and molecular dynamics simulations

Abstract

The structures of 15 La–Al–Si–O glasses, whose compositions span 11–28 mol% La₂O₃, 11–30 mol% Al₂O₃, and 45–78 mol% SiO₂, are explored over both short and intermediate length-scales by using a combination of solid-state ²⁷Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) simulations. MAS NMR reveals Al speciations dominated by AlO₄ groups, with minor but significant fractions of AlO₅ (5–10%) and AlO₆ (≲3%) polyhedra present in all La₂O₃–Al₂O₃–SiO₂ glasses; the amounts of Al^[5] and Al^[6] coordinations increase for decreasing molar fraction of Si. The MD simulations reproduce this compositional trend, with the fractional populations of AlO_p groups (p = 4, 5, 6) according well with the experimental results. The modeled La speciations mainly involve LaO₆ and LaO₇ polyhedra, giving a range of average La³⁺ coordination numbers between 6.0 and 6.6; the latter increases slightly for decreasing Si content of the sample. Besides the expected bridging and non-bridging O species, minor contributions of oxygen triclusters (≤9%) and free O²⁻ ions (≤4%) are observed in all MD data. The glass structures exhibit a pronounced Al/Si disorder; the MD simulations reveal essentially random SiO₄–SiO₄, SiO₄–AlO_p and AlO_p–AlO_q (p, q = 4, 5, 6) associations, including significant amounts of AlO₄–AlO₄ contacts, regardless of the n_Al/n_Si molar ratio of the glass. The strong violation of Al^[4]–Al^[4] avoidance is verified by 2D ²⁷Al NMR experimentation that correlates double-quantum and single-quantum coherences, here applied for the first time to aluminosilicate glasses, and evidencing AlO_p–AlO_q connectivities dominated by AlO₄–AlO₄ and AlO₄–AlO₅ pairs. The potential bearings from distinct fictive temperatures of the experimental and modeled glass structures are discussed.