Abstract
The crystal structures of dehydrated Pb/sub 6/-A (zeolite A fully Pb/sup 2 +/ exchanged at pH 4.3; a = 12.162 (2) A) and partially dehydrated Pb/sub 9/(OH)/sub 8/(H/sub 2/O)-A (zeolite A overexchanged with Pb/sup 2 +/ at pH 6.0; a = 12.225 (3) A) have been determined by single-crystal X-ray diffraction methods in the cubic space group Pm3m. The structures were refined to final R (weighted) indices of 0.078 and 0.088, respectively. In Pb/sub 6/-A, five Pb/sup 2 +/ ions lie on 3-fold axes where each associates with a 6-oxygen ring of the aluminosilicate framework; the sixth Pb/sup 2 +/ ion, perhaps a PbOH/sup +/ ion, lies in the plane of an 8-oxygen ring; an additional small fraction of a Pb/sup 2 +/ ion may be present. In Pb/sub 9/(OH)/sub 8/(H/sub 2/O)/sub 3/-A, Pb ions occupy four nonequivalent sites. Within the sodalite unit, two Pb/sup 2 +/ ions, each 2.69 A from three framework oxygens, are located opposite para 6-rings and are bridged by a nonframework oxygen. An ion which, by its coordinate geometry, appears to be Pb/sup 4 +/ is nearly in a 6-ring plane where it coordinates equatorially to three framework oxygens and axially to nonframework oxygens. Twomore » Pb/sup 2 +/ ions at general positions are near 8-ring planes; each is 4-coordinate to two 8-ring oxygens and to two nonframework oxygens, each at 2.54 A on either side of the 8-ring. The remaining four Pb/sup 2 +/ ions are arranged tetrahedrally around the ..cap alpha..=cage, each bonded to three 6-ring framework oxygens and 2.11 A from a nonframework oxygen, probably a OH/sup -/ ion. Altogether, three clusters, each containing three Pb ions and a few hydroxide ions, are found. They are ((Pb/sup 2 +/)/sub 3/(OH)/sub 2/(H/sub 2/O)/sub 2/)/sup 4 +/ and ((Pb/sup 2 +/)/sub 3/(OH)/sub 3/(H/sub 2/O))/sup 3 +/ in the large cavity, and ((Pb/sup 2 +/)/sub 2/Pb/sup 4 +/(OH)/sub 3/)/sup 5 +/ in the sodalite unit and extending through a single 6-ring. These structures were dehydrated under vacuum at 350 and 450/sup 0/C respectively; it is not clear why residual water oxygens were found in the second structure. 45 references, 4 figures, 4 tables.« less
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