Chlorine Nuclear Magnetic Resonance as a Sensitive Probe to Study Crystalline to Glasslike Interfaces in Calcium...

The calcium chlorapatite Ca₅(PO₄)₃Cl samples with controlled surface morphology ranging from low to high surface areas were studied using ^35,37Cl magic-angle spinning and static nuclear magnetic resonance (NMR). Complex ^35,37Cl NMR lineshapes obtained for all studied Ca₅(PO₄)₃Cl samples, consisting of two components, were analyzed using quadrupolar and Czjzek models that characterize highly crystalline and glasslike moieties, respectively. Transmission electron microscopy micrographs confirmed the formation of the glasslike material as a shell, while crystalline moieties were formed in the core of the particles. The ratio of crystalline to glasslike material was related to the morphology of the samples: smaller particles contained more glasslike species, while larger particles were composed mostly of crystalline material. The results obtained highlighted the importance of quadrupolar NMR as an experimental method for correlating the particle morphology with phase composition at the molecular level. Moreover, quadrupolar NMR demonstrated its robustness as a complementary technique to X-ray diffraction for the characterization of complex materials, especially those consisting of several coexisting phases, some of which lack long-range order, e.g., glasses and glasslike materials.