Crystal-fluid interactions in open-framework materials at high pressure

The number of experiments on the high-pressure behavior of open-framework materials increased significantly in the last decade [1,2]. The framework topology, the chemical composition and the so-called “host-guest” interactions (between the framework and the extraframework components) were found to significantly influence the response of zeolites to the applied pressure. However, the HP-behavior of zeolites may also be influenced by crystal-fluid interactions when P-transmitting fluids (PTF) are used to generate hydrostatic compression, and in particular when the PTF is “pore-penetrating”. In such a case, the P-induced penetration of PTF molecules into the zeolite structural voids leads to a change of the physical-chemical properties of the studied material, for example inducing a stiffening of the elastic behavior or leading to the hyperconfinement of supramolecular aggregates (in the zeolite channels) with functional properties [3]. In this study, we describe the HP-behavior and the crystal-fluid interactions of two synthetic zeolites with empty channels and cages, i.e. all-silica ferrierite (Si-FER) and ALPO4-5 (AlPO4), compressed with non-penetrating (silicone oil, s.o.) and potentially pore-penetrating PTF. The compression of Si-FER in s.o. evidences the remarkable flexibility of this framework: a first displacive phase transition was observed from the Pmnn to the P121/n1 space group at ~ 0.7 GPa. A second displacive phase transition, involving a significant unit-cell volume contraction, was observed at ~ 1.24 GPa from the P121/n1 to the P21/n11 space group (through an intermediate P-1 structure, “type-II” transition according to Christy [4]). The high-P P21/n11 polymorph was found to be stable at least up to 3.00(7) GPa, whereas - upon pressure release - the starting Pmnn structure was fully recovered. The three polymorphs were found to share a virtually identical bulk elastic behavior, being their average volume compressibility βV: 0.051(4), 0.056(9) and 0.055(3) GPa-1, respectively. The compression of Si-FER and ALPO-5 in potentially pore-penetrating PTF showed a lower bulk compressibility, different phase-transition paths (for Si-FER) and diverse atomic-scale deformation mechanisms with respect to the compression in silicone oil, suggesting the onset of significant crystal-fluid interactions, likely due to the P-induced penetration of PTF molecules. In addition, the HPbehavior of Si-FER is strongly influenced by the process kinetics, which was found to control the P-induced molecules intrusion phenomena and, as consequence, the P-induced phase transitions in this material. The authors acknowledge the Italian Ministry of Education, MIUR-Project: “Futuro in Ricerca 2012 - ImPACTRBFR12CLQD”. [1] G.D. Gatta, Y. Lee Mineral. Mag. 2014, 78, 267-291. [2] G. Vezzalini, R. Arletti, S. Quartieri Acta Cryst. 2014, B70, 444-471. [3] M. Santoro, F.A. Gorelli, R. Bini, J. Haines, A. Van der Lee Nat. Commun. 2013, 4, 1557-1563. [4] A.G. Christy Acta Cryst. 1993, B49, 987-996.