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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 15, Number 4
BKCSDE 15(4)
April 20, 1994 

 
Title
Crystal Structures of Zeolitic Water Molecules in Cs3Na9-A and Cs3Na8H-A
Author
Kee Heon Cho, Jin Hyun Kwon, Hae Won Kim, Chong Sam Park, Nam Ho Heo
Keywords
Abstract
Structures of zeolitic water molecules in hydrated Cs3Na9-A and Cs3Na8H-A have been studied by single-crystal x-ray diffraction methods in the cubic space group Pm3m at 21 ℃. In the crystal structure of fully dehydrated Cs3Na9-A (a = 12.265(1) Å with R1=0.056 and R2=0.057), nearly three and eight Cs+ and Na+ ions per unit cell are found at the centers of 8-and 6-oxygen rings with closest approaches of 3.401(11) and 2.303(6) Å to framework oxygens, respectively. The twelfth Na+ is located opposite 4-oxygen ring in the large cavity with an occupancy of 0.6(2). The crystal structures of hydrated Cs3Na9-A (a=12.273(l) Å with R1=0.065 and R2=0.078) and Cs3Na8H-A (a=12.286(l) Å with R1=0.078 and R2=0.081) are viewed as having two kinds of unit cells, Cs3Na9-xHx-A·20H2O and Cs3Na9-xHx-A·24H2O (x=0 and 1, respectively), each with different secondary structures of zeolitic water molecules in its large cavity. In both unit cells, Cs+ ions are found at centers of 8-rings with large thermal parameters, showing changes in their coordination environments due to approaches of water molecules upon hydration. Na+ ions on 6-rings have also changed their geometrics from trigonal to tetrahedron upon hydration, each with three framework oxygens at 2.356(11) Å and an additional zeolitic water molecule at 2.18(3) Å on a threefold axis which is perpendicular to the 6-ring of the framework oxygens. In the large cavity Of Cs3Na9-xHx-A·20H2O, four water molecules are arranged tetrahedrally on the threefold axes, each in trun with tetrahedrally coordinated three water molecules at 2.83(6) Å. In the case of Cs3Na9-xHx-A·24H2O, eight waters molecules on the threefold axes occupy eight symmetry equivalent positions, each in trun with tetrahedrally coordinated three waters at 2.51(6) Å, resulting in a pseudo-dodecahedron of twenty water molecules with twelve shared and distorted pentagons perturbed by approaches of Cs+ ions. Four water molecules in each sodalite unit of both crystals are arranged tetrahedrally and seem to have weaker interactions with the Na+ ions on 6-rings due to the stronger interactions between the Na+ ions and the water molecules in the large cavity.
Page
297 - 304
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