Gismondine under HP: Deformation mechanism and re-organization of the extra-framework species

The response to compression of the zeolite gismondine, which is the natural counterpart of the synthetic zeolite P, was explored by synchrotron X-ray powder diffraction experiments with a non-penetrating pressure-transmitting medium and by Car Parrinello molecular dynamics simulations. In the range P-amb-7.4 GPa, no pressure-induced amorphization occurs and the cell volume decrease is approximately 8%. The corresponding bulk modulus (K-0 = 63.8(2) GPa) is one of the highest found to date for zeolites studied under the same conditions. The pressure-induced cell modifications are found to be reversible upon decompression up to about 4 GPa. Gismondine is found to be more compressible along a and c with respect to b, and shows a tendency from monoclinicity towards tetragonality with increasing pressure. The results of the Molecular Dynamics simulations made it possible to rationalize at the microscopic level the slope variation observed in the volume-pressure curve and to explain the role of the framework and extra-framework atoms in the deformation mechanism. Upon compression, the Ca coordination number increases and the water molecules organize themselves in a different supra-molecular arrangement. Interestingly the pressure-induced deformation mechanism described here is similar to that found in gismondine upon dehydration under vacuum.