The crystal structures of vacuum-dehydrated fully Ca2+-exchanged zeolite A (a - 12.278 (2) Á) and vacuum-dehydrated fully Sr2+-exchanged zeolite A (a = 12.316 (2) Á) have been determined by single-crystal x-ray diffraction methods in the cubic space group Pmlm. The structures were refined to final R (weighted) indices of 0.066 and 0.096, respectively. In each structure, five divalent cations are located on threefold axes near the centers of 6-oxygen rings. In Ca^-A, the five Ca2+ ions are located at three unique equipoints: three equivalent ions are recessed 0.51 Á into the large cavity; one is nearly in the plane of a 6-oxygen ring; the fifth is 0.63 A into the sodalite unit. The five threefold-axis Sr2+ ions occupy two equipoints: three equivalent ions are recessed 0.28 Á into the large cavity, and two are 0.78 Á into the sodalite unit, on opposite sides of the origin. The sixth Ca2+ ion is located at an unusual position in the plane of an 8-oxygen ring, and the sixth Sr2+ ion occupies a similar site near this plane. This Ca2+ ion is 3.08 (4) Á from its nearest neighbor, a single oxide ion of the aluminosilicate framework-this exceeds the sum of the respective ionic radii by 0.77 Á. (Two other oxide ions are 3.13 (6) Á from this Ca2+ ion.) The corresponding Sr2+ ion is 2.96 (12) Á from its nearest neighbor, a framework oxide ion-this distance is 0.52 Á longer than the sum of the corresponding ionic radii. (Two other oxide ions are 2.98 (13) Á from this Sr2+ ion.) These discrepancies are especially significant because the conventional ionic radii used are appropriate for larger coordination numbers and should be too long to describe the low-coordinate situation of the 8-ring cation. Presumably, one ion is located in an 8-ring in each structure because this allows a more satisfactory distribution of positive charge than would be possible if all six cations per unit cell occupied threefold-axis sites.
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