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Two crystal structures of the vacuum dehydrated Ag+-exchanged zeolite X have been determined by single-crystal X-ray diffraction techniques in the cubic space group Fd3 at 21(1)℃ (a=24.922(1)Å and a=24.901(1)Å, respectively). Each crystal was ion exchanged in flowing streams of aqueous AgNO3 for three days. The first crystal was dehydrated at 300℃ and 2×10-6 torr for two days. The second crystal was similarly dehydrated at 350℃. Their structures were refined to the final error indices, R1=0.095 and R2=0.092 with 227 reflections, and R1=0.096 and R2=0.087 with 334 reflections, respectively, for which I > 3σ(I). In the first crystal, Ag species are found at five different crystallographic sites: sixteen Ag+ ions fill the site Ⅰ, the center of the double 6-ring, thirty-two Ag0 atoms fill the Ⅰ' site in the sodalite cavities opposite double six-rings, seventeen Ag+ ions lie at the 32-fold site Ⅱ' inside the sodalite cavity at the single six-oxygen ring in the supercage, fifteen Ag+ ions lie at the 32-fold site Ⅱ, in the supercage, and the remaining twelve Ag+ ions lie at site Ⅲ' in the supercage at a little off two-fold axes. In the second crystal, all Ag species are located similarly as crystal 1; 16 at site Ⅰ, 28 at site Ⅰ', 16 at site Ⅱ, 16 at site Ⅱ', 6 at site Ⅲ and 6 at site Ⅲ'. Total 88 silver species were found per unit cell. The remaining four Ag atoms were migrated out of the zeolite framework to form small silver crystallites on the surface of the zeolite single crystal. In the first structure, the numbers of Ag atoms per unit cell are approximately 32.0 and these may form tetrahedral Ag4 clusters at the centers of the sodalite cavities. The probable four-atom cluster is stabilized by coordination to two Ag+ ions. The Ag-Ag distance in the cluster, ca. 3.05 Å, is a little longer than 2.89 Å, Ag-Ag distance in silver metal. At least two six-ring Ag+ ions on sodalite cavity (site Ⅱ') must necessarily approach this cluster and this cluster may be viewed as a distorted octahedral silver cluster, (Ag6)2+.
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