Institute o[ Silicate Chemistry. of the Academy of Sciences of the USSR
Crystal structures of the hydrated forms of three partially K-, Rb-, and Cs-exchanged NaX [Si/AI = 1.11(2)] with the exchange levels of 0,57(6), 0.50(5i, and 0.48(4), respectively, and of the K and Cs dehydrated forms have been determined using X-ray single-crystal data. For the dehydrated forms, data collection was carried out at 400°C. The structures were refined in the cubic space group Fd3 to final R-factors of 0.031 (233 reflections), 0.045 (360) and 0.063 (656) for the hydrated and 0.078 (229) and 0.059 (376) for the dehydrated forms, respectively.
Analysis of the cation distribution in the hydrated forms shows that K ions penetrate into all zeolite cavities, whereas Rb ions diffuse into the sodalite cage, but not into the hexagonal prism, and Cs ions are located only in the supercage. Dehydration of the zeolites is accompanied by a migration of unexchanged sodium cations into the hexagonal prism. Destruction of the dehydrated Rb-exchanged crystal after the 6 h-long exposure at 400°C seems to be caused by a slow migration of the Rb ions into the prism up to a marked level. In d-CsNaX. Cs ions have been observed in the sodalite cage and, in minor amounts (about 1 per unit cell), in the prisim. Obviously, such a small concentration of Cs in the prism cannot cause a loss of crystallinity, as does the Rb in d-RbNaX. The cation content of site II in the partially exchanged X zeolites is strongly influenced by the kind of the exchanged cations.