FORMATION OF HEXASILVER AT THE CENTER OF THE LARGE CAVITY - 3 CRYSTAL-STRUCTURES OF DEHYDRATED AG+-EXCHANGED AND CA2+-EXCHANGED ZEOLITE A, AG12-2XCAX-A (X = 2, 3, AND 4) TREATED WITH RUBIDIUM VAPOR

Three crystal structures of dehydrated Ag+- and Ca2+-exchanged zeolite A (Ag4Ca4-A, Ag6Ca3-A, and Ag8Ca2-A) treated at 250-degrees-C with 0.1 Torr of Rb vapor have been determined by single-crystal X-ray diffraction techniques in the cubic space group Pm3m at 21 (1) degrees-C (a = 12.271 (1) angstrom, 12.255 (1) angstrom, and 12.339 (1) angstrom, respectively). Their structures were refined to the final error indices, R(weighted), of 0.072 with 130 reflections, 0.050 with 110 reflections, and 0.083 with 86 reflections, respectively, for which I > 3-sigma(I). In each structure, Rb species are found at three different crystallographic sites: 3 Rb+ ions per unit cell are located at 8-ring centers, ca. 5.6-6.4 Rb+ ions are found opposite 6-rings on threefold axes in the large cavity, and ca. 2.5-3.0 Rb+ ions are found on threefold axes in the sodalite unit. Also, Ag species are found at two different crystallographic sites: ca. 0.7-2.1 Ag+ lie opposite 4-rings and ca. 2.2-4.8 Ag atoms are located near the center of the large cavity. In these structures, the numbers of Ag atoms per unit cell are 2.2, 2.4, and 4.8, respectively, and these are likely to have formed hexasilver clusters at the centers of the large cavities. The Rb+ ions, by blocking 8-rings, may have prevented silver from migrating out of the structure. Each hexasilver cluster is stabilized by coordination to up to 13 Rb+ ions. An excess absorption of about 0.8 Rb atom per unit cell indicates the presence of a triangular symmetric (Rb3)2+ cation in the sodalite cavity. At least one large-cavity 6-ring Rb+ ion must necessarily approach this cluster and may be viewed as a member of it to give (Rb4)3+, (Rb5)4+, or (Rb6)5+.