Application of High-Resolution 95Mo Solid State NMR Spectroscopy in Structural Studies of Complex Alkali Molybdate Crystals and Glasses

Abstract

The Mo–O coordination environment is probed in a series of simple and complex crystalline alkali molybdates as well as in mixed-alkali molybdate glasses using high-field (18.8 and 20.0 T) ⁹⁵Mo magic-angle-spinning (MAS) and multiple-quantum MAS (MQMAS) nuclear magnetic resonance (NMR) spectroscopy. When taken together, the ⁹⁵Mo NMR spectroscopic results indicate that somewhat contrary to the conventional wisdom the corner- and edge-shared MoO₆ octahedral sites in these alkali molybdates are characterized by higher values of the isotropic shift (δ_iso) and quadrupolar coupling constant (C_Q) compared to the MoO₄ tetrahedral sites. These trends are hypothesized to be related to the unusually strong distortion of MoO₆ octahedra in corner- and edge-shared configurations and the resulting increase in the paramagnetic component of the chemical shift. While the ⁹⁵Mo C_Q of the MoO₄ sites displays an approximately linear positive correlation with the degree of tetrahedral distortion, no such correlation is observed for the MoO₆ sites. High-resolution ⁹⁵Mo NMR spectra show the coexistence of tetrahedral and octahedral Mo–O environments in the structure of alkali molybdate glasses, with the relative fraction of the latter environment increasing with Mo content. The results presented in this study indicate that high-resolution ⁹⁵Mo NMR spectroscopy at high magnetic fields (∼20 T or higher) may prove to be a promising tool for investigating the Mo–O coordination environments in nuclear waste glasses.