Synthesis and crystal structure of nitrate enclathrated sodalite Na-8[AlSiO4](6)(NO3)(2)

The hydrothermal formation of sodalites in the system Na2O-2SiO(2)-Al2O3-NaNO3-H2O has been investigated. Besides temperature and pressure, the initial materials of synthesis were varied to obtain the most perfect single crystals of nitrate enclathrated in sodalite. The alkaline transformation of Zeolite A as the starting aluminosilicate at a temperature of 648 K and a pressure of 0.1 GPa has been proved to be the best method for sodalite synthesis. A single crystal structure refinement, as well as MAS NMR of the nucleus Na-23 of nitrate sodalite, have been carried out to determine the position of the enclathrated nitrate anions in the open sodalite structure. The nitrogen atoms are located in an off-centre position within the sodalite cages. One of the three oxygen atoms of the nitrate group is positioned near the centre of a face of the tetrahedron formed by the sodium cations. The two remaining oxygen atoms of the nitrate group are located near the middle of the edges of the sodium tetrahedron. This arrangement of the nitrate anions is responsible for two split-positions of the sodium cations according to an occupancy ratio of 3:1.