Structural Role of Nb2O5 in Phosphate Glasses: An Advanced Solid-State NMR Protocol for the Glass System xNb2O5–(100–x)NaPO3

The structural role of Nb2O5 in oxide glasses remains poorly understood, despite the unique linear and nonlinear optical properties that it bestows. Here, advanced solid-state NMR methods can yield valuable insight, but their full potential has been underutilized for niobium-containing systems, especially in respect of the dipolar techniques that provide quantitative information on the interatomic connectivity and distance distributions. This study presents a new NMR-strategy and applies it to the model glass system xNb2O5–(100–x)NaPO3 (0 ≤ x ≤ 40). The number of P–O–P linkages per P atom is estimated from the 31P–31P dipole–dipole interactions using spin echo decay (SED) and double-quantum based dipolar recoupling effecting nuclear alignment reduction (DQ-DRENAR). The number of P–O–Nb linkages is obtained from the 31P–93Nb dipolar coupling using 93Nb{31P} rotational echo double resonance (REDOR) and, for the first time, 31P{93Nb} rotational echo saturation pulse double resonance (RESPDOR). Constrained by these interaction-selective experiments, which also include 31P/23Na double resonance spectroscopy, the poorly resolved 31P MAS NMR spectra are quantitatively decomposed into their contributions from the various network-forming units. Additional field dependent 93Nb MAS NMR experiments provide chemical shift parameters that reveal multiple six-coordinate niobium environments with varying degrees of distortion. Overall, the results demonstrate that Nb2O5 assumes a network former role, increasing the overall network connectivity. They also demonstrate an advanced solid-state NMR protocol for characterizing the structural role of Nb2O5, an important intermediate oxide, in multicomponent glasses.