Solid-state NMR and short-range order in crystalline oxides and silicates: a new tool in paramagnetic resonances

Abstract

Most applications of high-resolution NMR to questions of short-range order/disorder in inorganic materials have been made in systems where ions with unpaired electron spins are of negligible concentration, with structural information extracted primarily from chemical shifts, quadrupolar coupling parameters, and nuclear dipolar couplings. In some cases, however, the often-large additional resonance shifts caused by interactions between unpaired electron and nuclear spins can provide unique new structural information in materials with contents of paramagnetic cations ranging from hundreds of ppm to several per cent and even higher. In this brief review we focus on recent work on silicate, phosphate, and oxide materials with relatively low concentrations of paramagnetic ions, where spectral resolution can remain high enough to distinguish interactions between NMR-observed nuclides and one or more magnetic neighbors in different bonding configurations in the first, second, and even farther cation shells. We illustrate the types of information available, some of the limitations of this approach, and the great prospects for future experimental and theoretical work in this field. We give examples for the effects of paramagnetic transition metal, lanthanide, and actinide cation substitutions in simple oxides, pyrochlore, zircon, monazite, olivine, garnet, pyrochlores, and olivine structures.

NMR crystallography: structure and properties of materials from solid-state nuclear magnetic resonance observables

“…All examples described thus far are of diamagnetic solids. Stebbins and co-workers have reviewed the applications of high-resolution solid-state NMR to the study of silicate, phosphate, and oxide materials with relatively low concentrations of paramagnetic ions, where spectral resolution can remain high enough to distinguish interactions between NMRobserved nuclides and one or more magnetic neighbors in different bonding configurations in the first, second, and even farther cation shells (Stebbins et al, 2017). This work discusses the value and practical considerations associated with employing paramagnetic effects to gain structural information, in particular concerning short-range order.…”

Temperature dependent structure and dynamics in smectite interlayers: 23Na MAS NMR spectroscopy of Na-hectorite

“…Thus, there has been substantial effort devoted to understanding the structural, dynamical, and energetic aspects of the interaction of ions and water with geochemically important minerals 1-28 and materials of technological importance. [29][30][31][32] The temperature and pressure dependence of these interactions is complex and has been a long standing topic of discussion. [33][34][35][36][37][38] The smectite group of clay minerals has been of particular interest in geoscience, chemistry and materials science, because of their abundance and ability to intercalate a wide variety of organic and inorganic cations, along with water and other uid species.…”

Oxygen Evolution Reaction by Silicate-Stabilized Manganese Oxide

“…The interactions between unpaired electrons and nuclear spins are complicated. − In the presence of unpaired electrons, two types of interactions strongly result in peak positions. First, the Fermi contact shift, which involves the through-bond transfer of unpaired electron spin density from a paramagnetic cation to the Si atom.…”

“…The 29 Si NMR spectroscopy has been widely used for finding features for crystalline and glassy silicates. − However, the investigation of 29 Si NMR for materials with contents of cations with unpaired electrons, e.g. , the transition metal, is not easy since unpaired electrons can strongly interact with the observed nuclear spins and result in the broadening of spectra. − Different shifts for 29 Si NMR signals are also observed (−60 to −120 ppm), which are far outside the known range for Si in diamagnetic compounds.…”

“…29 Si NMR spectroscopy for 2 shows a complicated spectrum with a high shift and a complicated pattern for [SiO 4 ] units. Therefore, 29 Si NMR spectroscopy indicates that the Si–O groups chemically bond to Mn ions − (Figure d). We suggest that these Si–O–Mn bonds cause a force to inhibit the leaking of Mn ions from the surface of the catalyst.…”