The crystal structure of .BETA.-K4SiW12O40 9H2O, containing an isomer of the Keggin ion.
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概要
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β-K<SUB>4</SUB>SiW<SUB>12</SUB>O<SUB>40</SUB>·9H<SUB>2</SUB>O is orthorhombic, with the space group Pnma and <I>a</I>=20.62(1), <I>b</I>=15.57(1), <I>c</I>=12.95(1) Å, <I>D</I><SUB>m</SUB>=4.88 g cm<SUP>−3</SUP> and <I>D</I><SUB>x</SUB>=5.04 g cm<SUP>−3</SUP> for <I>Z</I>=4. The crystal contains a discrete β-SiW<SUB>12</SUB>O<SUB>40</SUB><SUP>4−</SUP> polyanion, a geometrical isomer of the α-SiW<SUB>12</SUB>O<SUB>40</SUB><SUP>4−</SUP> with the well known Keggin structure. The idealized Keggin ion has a T<SUB>d</SUB> symmetry, but the β-isomer has only a C<SUB>s</SUB> symmetry. Mutual transformation between the α- and β-structures is possible through a 60° rotation of one of the four W<SUB>3</SUB>O<SUB>13</SUB> units, which make a cage around a SiO<SUB>4</SUB> tetrahedron in both structures. The other possible geometrical isomers of the Keggin structure are discussed.
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