High pressure deformation mechanism of Li-ABW: Synchrotron XRPD study and ab initio molecular dynamics simulations

The response to compression of the synthetic zeolite Li-ABW (LiAlSiO4 center dot H2O, Z = 4, s.g. Pna2(1)) was explored by synchrotron X-ray powder diffraction experiments, using silicone oil as non-penetrating pressure transmitting medium, and Car Parrinello Molecular Dynamics simulations. In the range P-amb - 8.9 GPa, a nearly isotropic compression for the axial parameters and a cell volume decrease of approximately 12% are observed. A discontinuity in the cell parameters vs P behaviour can be detected between 5 and 6 GPa. As a consequence, the bulk modulus was calculated separately in the P-amb - 4.9 GPa and 5.6-8.9 GPa pressure ranges. The corresponding values (72(2) GPa and 80(2) GPa, respectively) are among the highest found up to now for zeolites studied with non-penetrating P-transmitting media. Molecular Dynamics simulations were performed at volumes corresponding to P-amb, 1.5, 5.6, and 7.6 GPa, respectively. At 1.5 GPa the channel system is already elliptically deformed, and the zig-zag trend of the 4-ring tetrahedral chains is enhanced. Moreover, the water molecule chain running along the channel becomes interrupted and the water molecules are more strongly connected to the framework oxygen atoms. The four-fold coordination of Li cation is maintained up to the highest pressure and only a slight bond distance decrease is observed above 1.5 GPa. In the P-amb - 5.6 GPa range, all T-O-T angles decrease with pressure, and hence the Li-ABW structure can be defined as collapsible. Otherwise, at higher compression, average T-O-T angles increase slightly. Overall, the deformation of the Li-ABW upon compression resembles that achieved by anhydrous Li-ABW in the high temperature regimes.