Powder XRD, SEM, and Multinuclear MAS‐NMR Investigations of the Interactions Between Glass and Crystalline Phases of Li, Na, or K Ceramic Waste Forms

Abstract

Interactions between the glass and crystalline phases of ceramic waste forms were investigated via powder X‐ray diffraction, scanning electron microscopy, and 29Si, 27Al, 23Na, 7Li, and 35Cl magic angle spinning nuclear magnetic resonance spectroscopy. LiCl, NaCl, or KCl waste form samples were made with or without glass. The waste forms containing glass consist of sodalite and glass phases with minor amounts of nepheline. Samples without glass form varying amounts of sodalite and nepheline. The glass frit, intended to bind the zeolite particles together, changes in composition, showing marked increases in aluminum and alkali content.

Highly Photoluminescent Sulfide Clusters Confined in Zeolites

“…From the XRD patterns, it is possible to calculate the lattice parameters from the crystalline sodalite and nepheline structure using Bragg’s Law, considering that sodalite and nepheline form cubic and hexagonal structures, respectively. While the sodalite phase presents lattice parameters of approximately a = 8.85 Å, the nepheline displays a = 9.85 Å and c = 8.32 Å (Table ), which once more are in agreement with values reported in the literature. , …”

“…For higher S/Cl molar ratios, a nepheline crystalline phase formation is favoured due to higher sulphur contents. As it was already mentioned, anions are left out from the nepheline cavities in rich sulphur samples, 33,40,41 which corresponds to a situation where the overall sulphur and chlorine intercalation of the sample decreases (see Fig. 6).…”

“…32 In the sodalite zeolite structure the sodium atoms are coordinated to 3 oxygen atoms of the six-membered rings, and they are available to interact with anionic species. 33 Chang and co-workers have already reported that the occupation of the cavities of LTA zeolites with ions such as sodium and ionic halogens, when submitted to a heating process, induces the transformation of the initial structure to a sodalite zeolite topology. In this transformation a rotation of the tetrahedral structure occurs, so that Na + is coordinated to the oxygen ions of the zeolite framework and with other anions, such as chlorides.…”

“…Consequently, the conversion between such crystalline structures can be easily achieved at the synthesis conditions utilized in this study, as reported by Dimitrijevic and collaborators. 39 In addition, the nepheline and sodalite crystalline phases are related by the following reversible reaction (b): 33,40,41 Hence, the Na-LTA transformation to nepheline phase, when high sulphur contents are employed can occur (i) directly from Na-LTA, 39,42 or (ii) with sodalite as an intermediate phase. In order to better understand this transformation we have carried XRD experiments using different heating times during the thermal treatment of the sample containing an S/Cl nominal molar ratio of 2.0, where the nepheline phase is intensely present (XRD patterns are presented as supplementary material SI-8).…”

“…While the sodalite phase presents lattice parameters of approximately a = 8.85 Å, the nepheline displays a = 9.85 Å and c = 8.32 Å (Table 1), which once more are in agreement with values reported in the literature. 33,45 Moreover, the sodalite unit cell has a small volume increase with respect to the sulfur content (Table 1). This can be an indication of the confinement of sulfur within the sodalite cages in the form of S 2 − , which has a slightly larger ionic radius than Cl − (1.84 and 1.81 Å, respectively), or as polysulfide clusters.…”