Intermediate Role of Gallium in Oxidic Glasses: Solid State NMR Structural Studies of the Ga2O3–NaPO3 System

Abstract

A series of (NaPO3)1–x(Ga2O3)x glasses (0 ≤ x ≤ 0.35) prepared by conventional melt-quenching methods has been structurally characterized by various complementary high resolution one-dimensional and two-dimensional (2D) solid state magic angle spinning nuclear magnetic resonance (MAS NMR) techniques, which were validated by corresponding experiments on the crystalline model compounds GaPO4 (quartz) and Ga(PO3)3. Alloying NaPO3 glass by Ga2O3 results in a marked increase in the glass transition temperature, similar to the effect observed with Al2O3. At the atomic level, multiple phosphate species QnmGa (n = 0, 1, and 2; m = 0, 1, 2, and 3) can be observed. Here n denotes the number of P–O–P and m the number of P–O–Ga linkages, and (m + n ≤ 4). For resolved resonances, the value of n can be quantified by 2D J-resolved spectroscopy, refocused INADEQUATE, and a recently developed homonuclear dipolar recoupling method termed DQ-DRENAR (double-quantum based dipolar recoupling effects nuclear alignment reduction). Ga3+ is dominantly found in six-coordination in low-Ga glasses, whereas in glasses with x > 0.15, lower-coordinated Ga environments are increasingly favored. The connectivity between P and Ga can be assessed by heteronuclear 71Ga/31P dipolar recoupling experiments using 71Ga{31P} rotational echo double resonance (REDOR) and 31P {71Ga} rotational echo adiabatic passage double resonance (READPOR) techniques. Up to x = 0.25, the limiting composition where this is possible, the second coordination sphere of all the gallium atoms is fully dominated by phosphorus atoms. Above x = 0.25, 71Ga static and MAS NMR as well as REDOR experiments give clear spectroscopic evidence of Ga–O–Ga connectivity. 31P/23Na REDOR and REAPDOR results indicate that gallium has no dispersion effect on sodium ions in these glasses. They also indicate significant differences in the strength of dipolar interactions for distinct QnmGa species, consistent with bond valence considerations. On the basis of these results, a comprehensive structural model is developed. This model explains the compositional trend of the glass transition temperatures in terms of the concentration of bridging oxygen species (P–O–P, P–O–Ga, and Ga–O–Ga) in these glasses. The results provide new insights into the role of Ga2O3 as an intermediate oxide, with features of both network modifier and network former in oxide glasses.