Synthesis and crystal structures of gallium and germanium variants of cancrinite

A synthetic alummogermanate 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 P6₃ and a = 12.968(1), c = 5.132(1) angstroms, V = 747.4(1) angstroms³. 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 P6₃mc and a = 12.950(2), c = 5.117(1) angstroms, V = 743.2(2) angstroms³. 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 P6₃ reveals that the intensities of the hh2̄h̄l reflections with l = 2n+1 are less than 0.07% of the strongest (0 0 0 2) reflection, suggesting that P6₃ is probably the true space group. Resonant diffraction studies performed in the vicinity of the Ga K-edge confirmed the presence of the hh2̄h̄l reflections with l = 2n+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 6₃-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.