Effect of Sodium Oxide on Structure of Lanthanum Aluminosilicate Glass

Rare earth (RE) aluminosilicate glasses exhibit several favorable chemical, mechanical and thermal properties. As such, they are considered to be model systems for long-half-life actinides and are candidate containment materials for long-term immobilization of radioactive wastes. The aim of the present study was to investigate the effect of the substitution of sodium oxide on the glass transition temperature and structure of lanthanum aluminosilicate glasses. The primary objective was to elucidate the relationship between the substitution of Na₂O for La₂O₃ on the Tg reduction and structural characteristics of lanthanum aluminosilicate glass, including identifying changes in the main Qn species and local environments of Si and Al. The structure of SiO₂–Al₂O₃–La₂O₃–Na₂O glasses has not been studied previously, and, thus, this investigation is the first to assess the structural changes occurring when La₂O₃ is substituted by Na₂O. Three glasses were prepared with general composition (mol.%): 55SiO₂–25Al₂O₃–20M₂O_n (M = La or Na; n = 3 or 1). Glass G1 contains 20 mol.% La₂O₃; in G2, 15 mol.% of La₂O₃ was substituted by 15 mol.% Na₂O; and Glass G3 contains 20 mol.% Na₂O. The glasses were characterized by DSC to determine glass transition temperatures. As expected, as Na is substituted for La, T_g decreases substantially. Structural studies were carried out by FTIR spectroscopy,²⁹Si, and²⁷Al MAS NMR. As Na is substituted for La in these aluminosilicate glasses, the main goals that were achieved were the identification of Qⁿ species and also changes in the local environments of Si and Al: {QⁿSi(mAl)} and {QⁿAl(mSi)}.