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The structures of fully dehydrated Ca2+- and Cs+-exchanged zeolite X, Ca35Cs22Si100Al92O384(Ca35Cs22-X; a=25.071(1)Å) and Ca29Cs34Si100Al92O384(Ca29Cs34-X; a=24.949(1)Å), have been determined by single-crystal X-ray diffraction methods in the cubic space group Fd3 at 21(1)℃. Their structures were refined to the final error indices R1=0.051 and R2=0.044 with 322 reflections for Ca35Cs22-X, and R1=0.058 and R2=0.055 with 260 reflections for Ca29Cs34-X; I > 3σ(I). In both structures, Ca2+ and Cs+ ions are located at five different crystallographic sites. In dehydrated Ca35Cs22-X, sixteen Ca2+ ions fill site Ⅰ, at the centers of the double 6-rings(Ca-O=2.41(1)Å and O-Ca-O=93.4(3)°). Another nineteen Ca2+ ions occupy site Ⅱ (Ca-O=2.29(1) Å, O-Ca-O=118.7(4)°) and ten Cs+ ions occupy site Ⅱ opposite single six-rings in the supercage; each is 1.95Å from the plane of three oxygens (Cs-O=2.99(1)Å and O-Cs-O=82.3(3)°). About three Cs+ ions are found at site Ⅱ', 2.27Å into sodalite cavity from their three-oxygen plane (Cs-O=3.23(1)Å and O-Cs-O=75.2(3)°). The remaining nine Cs+ ions are statistically distributed over site Ⅲ, a 48-fold equipoint in the supercages on twofold axes (Cs-O=3.25(1)Å and Cs-O=3.49(1)Å). In dehydrated Ca29Cs34-X, sixteen Ca2+ ions fill site Ⅰ (Ca-O=2.38(1)Å and O-Ca-O=94.1(4)°) and thirteen Ca2+ ions occupy site Ⅱ (Ca-O=2.32(2) Å, O-Ca-O= 119.7(6)°). Another twelve Cs+ ions occupy site Ⅱ; each is 1.93Å from the plane of three oxygens (Cs-O=3.02(1)Å and O-Cs-O=83.1(4)°) and seven Cs+ ions occupy site Ⅱ'; each is 2.22Å into sodalite cavity from their three-oxygen plane (Cs-O=3.21(2)Å and O-Cs-O=77.2(4)°). The remaining sixteen Cs+ ions are found at Ⅲ site in the supercage (Cs-O=3.11(1) Å and Cs-O=3.46(2) Å). It appears that Ca2+ ions prefer sites Ⅰ and Ⅱ in that order, and that Cs+ ions occupy the remaining sites, except that they are too large to be stable at site Ⅰ.
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