Synthesis and crystal structures of gallium- and germanium-variants of the fibrous zeolites with the NAT, EDI and THO structure types

Two synthetic gallosilicates and a gallogermanate with the NAT, EDI and THO framework topologies have been synthesized under hydrothermal conditions and characterized by single crystal synchrotron X-ray diffraction. K-GaSi-NAT, K₈Ga₈Si₁₂O₄₀ · 6H₂O, is tetragonal, space group I42d, with a = 13.639(2), c = 6.545(1) Å. The framework model shows complete disordering of Ga and Si in tetrahedral sites, which is analogous to tetranatrolite but contrasts with the partial ordering in Na-GaSi-NAT. The T-sites of RbNa-GaSi-EDI, Rb₇NaGa₈Si₁₂O₄₀ · 3H₂O, exhibit partial disordering of Ga and Si in space group P42₁c. This leads to a cell doubling along the chain axis (c) with a = 9.773(1), c = 13.141(3) Å, a super cell modification of the Na-exchanged K-F structure. In Rb-GaGe-THO, Rb₂₀Ga₂₀Ge₂₀O₈₀ · 15H₂O, a = 14.335(3), b = 14.198(3), c = 13.421(3) Å, complete ordering on both tetrahedral and extra-framework sites lowers the symmetry from Pncn to the acentric space group Pn2n. An inspection of framework T-O-T bond angles, which are related to rotation and distortion of the chains, explains the differences in unit cells between these gallium- and germanium-variants and aluminosilicate analogs. The elliptical 8-ring windows, generated by four crosslinked chains in RbNa-GaSi-EDI and Rb-GaGe-THO, are the sites for (Rb, Na) and Rb cations, respectively, while the helical 8-ring channels formed in K-GaSi-NAT host only water molecules. The T₁₀O₂₀ windows, built by two neighboring chains, provide sites for K or Rb cations in each model. Mechanisms are proposed for cation-framework interactions, which are in turn responsible for the observed framework models. (C) 2000 Elsevier Science B.V. All rights reserved.