Anisotropic compression of a synthetic potassium aluminogermanate zeolite with gismondine topology

Compression behaviour of a potassium aluminogermanate with a gismondine framework topology (K-AlGe-GIS) was studied using in-situ high-pressure synchrotron X-ray powder diffraction. In contrast to the potassium gallosilicate analogue (K-GaSi-GIS), no elastic anomaly due to pressure-induced hydration and/or cation relocation was observed in K-AlGe-GIS. The Birch–Murnaghan fit to the pressure–volume data results in a bulk modulus of B0=31(1) GPa. The derived linear-axial compressibilities (i.e., βa=0.0065(5) GPa−1, βb=0.0196(4) GPa−1, βc=0.0081(7) GPa−1) indicate that the b-axis, normal to the 8-ring channels, is about three times more compressible than the a and c axes, parallel to the elliptical 8-ring channels. As a consequence a gradual flattening of the so-called ‘double crankshaft’ structural building units of the gismondine framework is observed. In K-AlGe-GIS, this flattening occurs almost linear with pressure, whereas it is nonlinear in the GaSi-analogue due to structural changes of the water–cation assembly under hydrostatic pressures.