Extra-Framework Anion Species in Scapolites Exhibiting Natural and Thermally Induced Photoluminescence: Insights from Single-Crystal XRD

This study presents results from comprehensive investigations of scapolite samples exhibiting natural or thermally induced photoluminescence. A range of analytical techniques, including single-crystal X-ray diffraction, Fourier-transform infrared spectroscopy, Raman spectroscopy, photoluminescence spectroscopy and electron paramagnetic resonance spectroscopy, were employed to characterize sulfur-related defect centers within the scapolite anionic cages. Differential scanning calorimetry combined with thermogravimetry and mass spectrometry was used to analyze the stages of degassing and quantify the volatile components. A new interpretation of the electron density distribution within the scapolite anionic cage is proposed. This enables the localization of possible precursors of S2•− radicals, the latter acting as luminescent centers. Key parameters, including atomic positions, occupancies, and bond lengths, were determined for the newly observed moieties. Furthermore, a novel methodological approach was developed to quantify SO42− and CO32− ratios in the structure based on the occupancy of their oxygen sites. Finally, the conditions necessary for the formation of persistent S2•− radicals in fluorescent scapolites are discussed, with an emphasis on the required cationic environment and charge-transfer conditions.