Compressibility and crystal–fluid interactions in all-silica ferrierite at high pressure

• The high-P behavior of Si-FER is studied by single-crystal and powder diffraction.
• Used P-media: silicone oil, “m.e.w.”, ethylene glycol and 2methyl-2propen-1ol.
• Compression in silicone oil showed two phase transitions Pmnn → P121/n1 → P21/n11.
• Compression in pore-penetrating fluids suggests P-induced intrusion of molecules.
• Several partially occupied sites can be assumed for intruded molecules in FER voids.

The high-pressure behavior of a synthetic siliceous ferrierite has been studied by in situ single-crystal and powder synchrotron X-ray diffraction with a diamond anvil cell, using four different P-transmitting fluids: the non-penetrating silicone oil and the potentially pore-penetrating methanol:ethanol:H2O = 16:3:1 mixture, ethylene glycol and 2methyl-2propen-1ol. The high-pressure experiment in silicone oil shows a remarkable flexibility of the FER framework. Two displacive phase transitions, following the path Pmnn-to-P121/n1-to-P21/n11 with pressure, were observed. The three polymorphs were found to share a virtually identical bulk compressibility, though showing a different anisotropic pattern. The experiments with potentially penetrating media enhanced the occurrence of a complex scenario, from which the P-induced intrusion of fluid molecules into the FER structural voids can be assumed by the different phase-transition paths and compressibility patterns, by the calculated residual electron density and by the different deformation mechanisms at the atomic scale, observed as a function of the used medium. The starting orthorhombic polymorph was always restored upon decompression in all the experiments. The roles of the different surface area in single crystal and polycrystalline samples, and of the process kinetics on the compressibility and crystal–fluid interactions, are discussed.

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