Synthesis, X-ray diffraction and MAS NMR characteristics of nitrate cancrinite Na-7.6[AlSiO4](6)(NO3)(1.6)(H2O)(2)

Investigations on the hydrothermal formation and the crystal structure of n itrate cancrinite have been carried out in the system Na2O-xSiO(2)-Al2O3-Na NO3-H2O, 1 < x < 6. Various starting materials have been examined to find t he best reaction system for cancrinite formation under low temperatures and pressures (473 K, autogenous pressure). Additional experiments were perfor med at elevated conditions at 773 K and 0.15 GPa. Polycrystalline nitrate c ancrinite and various amounts of unchanged educts were obtained in all low- temperature experiments. In contrast, in the high-temperature/high-pressure experiments a crossover of sodalite and cancrinite formation was observed as a function of the Si/Al ratio within the starting system for x = 1.0, 2. 0, 4.0 and 6.0 (i.e. at Si/Al ratios of 0.5, 1.0, 2.0 and 3.0). Structural investigations of nitrate cancrinite were performed by IR and MAS NMR spect roscopy of the nuclei Si-29, Al-27 and Na-23. The results confirmed the alt ernating Si, Al ordering of the alumosilicate framework for a Si/Al ratio o f 1.0 for all samples, independent of the Si/Al ratio of the educts as well as the temperature and pressure of crystallization. A distribution of the quadrupolar interaction of the sodium cations caused by the enclathrated wa ter molecules and motional effects can be suggested from the Na-23 MAS NMR spectrum. The single line with its slightly asymmetric shape makes it impos sible to distinguish the sodium cations inside the structure channel from t hose located in the epsilon-cages of the cancrinite structure. In order to complete the study on nitrate cancrinite a single crystal structure refinem ent was carried out. As in the case of carbonate cancrinite, NO3- anions ar e distributed over two nonequivalent positions in the hexagonal channel of the framework. All the crystal water molecules were found to be arranged in the a-cages of the cancrinite structure. (C) 2000 Elsevier Science S.A. Al l rights reserved.