Mixed Halide Sodalite Solid Solution Systems. Hydrothermal Synthesis and Structural Characterization by Solid State NMR

The solid solution behavior of mixed halide (Cl/Br, Cl/I, and Br/I) sodalite systems as prepared by hydrothermal synthesis has been studied in detail by X-ray diffraction and solid-state nuclear magnetic resonance (NMR). At a synthesis temperature of 450 K the rate of halide incorporation into the cages of mixed Cl/Br, Br/I, and Cl/I sodalites depends on the halide stoichiometry in the precursor gel and on the particular halogen anion. Na-23 and Al-27 NMR spectra acquired with magic angle spinning (MAS) are sensitive to both halide composition and distribution and can be used to monitor the onset of phase separation. The Cl/Br and Br/I sodalites show close to ideal miscibility behavior, whereas in the Cl/I sodalites domain segregation effects prevail. The NMR spectra of the quadrupolar Na-23 and the halogen nuclei also serve as sensitive indicators of the local symmetry perturbations caused by the disorder inherent to solid solution systems. Furthermore, the continuous variation of lattice parameters in sodalite solid solutions provides an opportunity to probe the fundamental dependence of chemical shift on Na-halogen distances. In this respect the experimental Na-23 NMR data reveal an exponential relationship, in agreement with theoretical calculations based on the Hartree-Fock method. In contrast, for the halogen chemical shifts the correlation is found to be approximately linear, with slopes of 17, 35, and 75 ppm/ Angstrom for Cl-35, Br-81, and I-127, respectively. All of these results reflect the high sensitivity of chemical shifts to the extent of anion-cation overlap for nuclei in ionic environments.