Synthesis and crystal structures of gallium and germanium variants of cancrinite

Abstract

A synthetic aluminogermanate and a gallogermanate with the Cancrinite group (CAN) framework topology have been synthesized under hydrothermal conditions and characterized by single crystal synchrotron X-ray diffraction. ALGe-CAN, Na6Cs2Al6Ge6O24. Ge(OH)(6), is hexagonal, with the space group P6(3) and a = 12.968(1), c = 5.132(1) Angstrom, V = 747.4(1) Angstrom(3). The T-sites exhibit complete ordering of Al and Ge atoms, similar to the framework models of aluminosilicate analogues. GaGe-CAN, Na6Cs2Ga6Ge6O24. Ge(OH)(6), is hexagonal, apparently with the space group P6(3)mc and a = 12.950(2), c = 5.117(1) Angstrom, V = 743.2(2) Angstrom(3). Although the observed data are consistent with the presence of the c-glide and consequent disordering of Ga and Ge atoms at the T-sites, calculation using a DLS-optimized framework in the space group P6(3) reveals that the intensities of the hh<(2h)over bar>l reflections with l = 2n + 1 are less than 0.07% of the strongest (0002) reflection, suggesting that P6(3) is probably the true space group. Resonant diffraction studies performed in the vicinity of the Ga K-edge confirmed the presence of the hh<(2h)over bar>l reflections with l2n + 1 and thus confirmed the ordering of the framework Ga/Ge atoms in GaGe-CAN. Inspection of the framework T-O-T bond angles demonstrates greater relative cell contraction for GaGe-CAN compared to AlGe-CAN and aluminosilicate counterparts. In both the structural models, Ge(OH)(6) octahedra are occluded in the 12-ring channels running along the 6(3)-axes. The sodium cations fully occupy the sites above the 6-ring windows in the 12-ring channels. The cesium cations fully occupy the sites in the middle of the cancrinite cages. Subtle differences in the coordination geometries of the extra-framework species are found, perhaps due to the pseudo-symmetry of GaGe-CAN. Thermogravimetry results indicate net weight losses of 3.5% and 3.0% for AlGe-CAN and GaGe-CAN, respectively, which are explainable by the dehydration of the Ge(OH)6 octahedra. In situ synchrotron X-ray powder diffraction demonstrated the formation of GaGe analogue of the nepheline hydrate I type structure at the temperature of complete dehydration. (C) 2000 Elsevier Science B.V. All rights reserved.