New Insights into the Cs Adsorption on Montmorillonite Clay from ¹³³Cs Solid-State NMR and Density Functional Theory Calculations

Abstract

The adsorption sites of Cs on montmorillonite clays were investigated by theoretical 133Cs chemical shift calculations, 133Cs magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectroscopy, and X-ray diffraction under controlled relative humidity. The theoretical calculations were carried out for structures with three stacking variations in the clay layers, where hexagonal cavities formed with Si–O bonds in the tetrahedral layers were aligned as monoclinic, parallel, alternated; with various d-spacings. After structural optimization, all Cs atoms were positioned around the center of hexagonal cavities in the upper or lower tetrahedral sheets. The calculated 133Cs chemical shifts were highly sensitive to the tetrahedral Al (AlT)-Cs distance and d-spacing, rather than to the Cs coordination number. Accordingly, three peaks observed in our theoretical spectra were interpreted to be adsorbed Cs around the center of hexagonal cavity with or without AlT and on the surface in the open nanospace. In a series of 133Cs MAS NMR spectral changes for partial Cs substituted samples, the Cs atoms are preferentially adsorbed at sites near AlT for low Cs substituted montmorillonites. The presence of nonhydrated Cs was also confirmed in partially Cs substituted samples, even after being hydrated under high relative humidity.

Multimodal Luminescent Yb3+/Er3+/Bi3+‐Doped Perovskite Single Crystals for X‐ray Detection and Anti‐Counterfeiting

“…The doping of paramagnetic Yb 3+ and Er 3+ ions induces a shift of the 133 Cs peak from 103.3 to 101.1 ppm as well as a significant broadening of this signal (the full-width-athalf-height increases from 173 to 300 Hz) ( Figure 1d). [42,43] The shift of peak position is due to the delocalization of extra electron density of Yb 3+ and Er 3+ (spin I = 1/2 and 3/2) on Cs + , that is, the Fermi-contact interaction. The broadening of NMR peak corresponds to the accelerated transverse (T 2 ) relaxation of 133 Cs in the presence of adjacent paramagnetic Yb 3+ and Er 3+ ions, resulting from the strong dipolar coupling between unpaired electrons and 133 Cs nuclei (i.e., the pseudo-contact interaction).…”

New Approach To Understanding the Experimental 133Cs NMR Chemical Shift of Clay Minerals via Machine Learning and DFT-GIPAW Calculations

“…The second-order quadrupolar interaction affects the line shape of the 133 Cs MAS NMR spectra, meaning that the peak position is not consistent with δ iso . However, the small quadrupole moment of 133 Cs leads to a negligibly small second-order quadrupolar interaction, allowing the determination of δ iso at the peak position …”

“…These structural differences can be identified by the coordination number inferred from the EXAFS spectra of adsorbed cations. In addition, adsorption sites in the nanopore space, which are formed by a layer mismatch or defects in the crystal structure, form new adsorption sites within the system. ,,, …”

“…SSNMR can detect detailed structural differences of 133 Cs, but the NMR parameters in relation to local structures are not yet fully understood. A reasonable degree of success has been achieved by comparing experimental data with NMR calculations based on model structures. − For example, a linear relationship between the interlayer distance and 133 Cs δ iso has been reported from a series of model structures with various interlayer spaces . The chemical shift is toward the low-field state due to the neighboring negatively charged Al sites on a tetrahedral sheet.…”

Cs Adsorption and CsCl Particle Formation Facilitated by Amino Talc-like Clay in Aqueous Solutions at Room Temperature

“…The strong interactions between the Cs ion and clay minerals have been widely studied through field studies, , laboratory experiments, and computational studies. − One study demonstrated the formation of the frayed edge site in the interlayer space caused by the insertion and subsequent dehydration of the hydrated Cs ion. It is one of the essential preconditions for the promotion of strong interaction between Cs and clay, apart from the interaction between the Cs ion and basal and edge surfaces .…”