Phase Transformations in the GIS Structure upon Ion Exchange

Important details of the dynamics of ion exchange are available only from time-resolved diffraction studies. In order to introduce contrast between T-sites in aluminosilicate ion exchangers we synthesized AlGe-analogues of the gismondine-related phases, Na- (Na ₂₄Al ₂₄Ge ₂₄O ₉₆*40H ₂O) and K-AlGe-GIS (K ₈Al₈Ge₈O₃₂*8H₂O) materials, which crystallize in space groups C2/c (a=14.490(3) Å, b=9.940(2) Å, c=23.530(5) Å ,β =105.90(1)° ), and I2/a (a=10.311(2) Å, b=9.749(1) Å, c=10.225(6) Å, β =90.000(2)° ), respectively. These unit cells (I2/a cell to C2/c cell) are related by the transformation a'= a - c; b'=b; c'= -a - 2c with a shift in origin of (1/4a + 1/2c). Noting the appearance and disappearance of ordering (C2/c) reflections, indicating the presence of the larger ordered cell for Na-GIS, were used to monitor the exchange process. At approximately 50% exchange the C2/c cell transforms to the smaller I2/a cell, with a disordered arrangement of cations in the channels. Le Bail extraction was performed to accurately determine the unit cell parameters. Single crystal diffraction experiments were also done to check the cell parameters and unit cell contents.