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.