The reactions of cesium with dehydrated calcium zeolite A: Crystal structures of Cs6Ca3-A, Cs11Ca0.5-A·0.5 Cs, and Cs12-A·0.5 Cs

Abstract

The Ca2+ ions in fully dehydrated, fully Ca2+-exchanged zeolite A can be reduced by cesium vapor. The extent of reaction depends upon experimental conditions. The structures of the products have been determined by single-crystal X-ray diffraction methods in the cubic space group Pm3m. With a very low cesium vapor pressure (ca. 10−3 Torr), dehydrated Ca6-A reacted with only 6 Cs atoms per unit cell at 350°C to yield Cs6Ca3-A (a = 12.240(2) with R1 = 0.085 and R2 = 0.057). When the vapor pressure of Cs0 was increased to 0.1 Torr at a lowered reaction temperature, 248°C, almost all Ca2+ ions were reduced except for one per two unit cells, resulting in an average unit cell composition of Cs11Ca0.5-A·0.5Cs (a = 12.207(2), R1 = 0.080, and R2 = 0.060). Increasing both the concentration of Cs0 (1.0 Torr) and temperature (304°C) led to a complete reaction: Cs12-A·0.5Cs (a = 12.241(1), R1 = 0.11, and R2 = 0.089). In these three crystal structures, cesium ions are located at four crystallographically distinct positions. Three Cs+ ions per unit cell were found at the centers of 8-rings, with, respectively, 2, 6, and 7 Cs+ ions on threefold axes in large cavities and 1, 2, and 2 Cs+ ions on threefold axes in sodalite units for the three crystals. In addition, in the latter two crystals, 0.5 Cs+ ions were found opposite 4-rings. These two crystals are presumed to contain the cesium clusters (Cs3)2+ and (Cs4)3+, respectively. The zeolite has avoided cation crowding in its cavities by absorbing extra cesium atoms. All unreacted Ca2+ ions are located at the centers of 6-rings, and Ca metal is seen on the crystal surface where it formed.