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., β_a=0.0065(5) GPa⁻¹, β_b=0.0196(4) GPa⁻¹, β_c=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.