Zeolites crystallize in a variety of three-dimensional structures in which oxygen atoms are shared between tetrahedra containing silicon and/or aluminium, thus yielding negatively charged tetrahedral frameworks that enclose cavities and pores of molecular dimensions occupied by charge-balancing metal cations and water molecules. Cation migration in the pores and changes, in water content associated with concomitant relaxation of the framework have been observed in numerous variable-temperature studies, whereas the effects of hydrostatic pressure on the structure and properties of zeolites are less well explored. The zeolite sodium aluminosilicate natrolite was recently shown to undergo a volume expansion at pressures above 1.2 GPa as a result of reversible pressure-induced hydration; in contrast, a synthetic analogue, potassium gallosilicate natrolite, exhibited irreversible pressure-induced hydration with retention of the high-pressure phase at ambient conditions. Here we report the structure of the high-pressure recovered phase and contrast it with the high-pressure phase of the sodium aluminosilicate natrolite. Our findings show that the irreversible hydration behaviour is associated with a pronounced rearrangement of the non-frame-work metal ions, thus emphasizing that they can clearly have an important role in mediating the overall properties of zeolites.
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Acknowledgements We thank J. Hu and the Geophysical Laboratory of the Carnegie Institute for access to their ruby laser system at beamline X17C. This work was supported by a Laboratory Directed Research and Development grant from Brookhaven National Laboratory (BNL) (Pressure in Nanopores). J.H. acknowledges financial support from the Royal Society, and J.P. thanks NSF and the American Chemical Society—Petroleum Research Fund. Research performed in part at the NSLS at BNL is supported by the US DOE, Division of Materials Sciences.
Acknowledgements We thank the TTF team at DESY, especially P. Castro, M. Minty, D. Nölle, H. Schlarb and S. Schreiber, for running the accelerator; we also thank J. R. Schneider for support and discussions. The first group of authors (H.W. to T.M.) built the apparatus for the cluster experiment and performed the experiment; the second group of authors (B.F. to M.Y) worked on the FEL and the diagnostics. We thank K.H. Meiwes-Broer, T. Brabec, C. Rose-Petruck, J. Krzywinski, M. Lezius, I. Kostyukov, J.M. Rost, E. Rühl, U. Saalman, J. Jortner and M. Smirnov and their research groups for discussions and comments, and J. Sutter for critically reading the manuscript. This work was supported by the DFG.
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