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Pressure-induced over-hydration and water ordering in gismondine: A synchrotron powder diffraction study

¹ Dipartimento di Scienze della Terra, Università di Modena e Reggio Emilia, Via S. Eufemia 19, 41100 Modena, Italy
² Dipartimento di Scienze della Terra, Università di Messina, Salita Sperone 31, 98166 Messina S. Agata, Italy
³ Swiss-Norwegian Beam Lines at ESRF, BP220, 38043 Grenoble Cedex, France

Correspondence: ^* E-mail: simona.quartieri{at}unimore.it

This paper reports the results of an in situ HP synchrotron X-ray powder diffraction investigation on the natural zeolite gismondine (ideal chemical formula Ca₄Al₈Si₈O₃₂·16H₂O, space group P2₁/c). The study was performed from P_amb to 7.9 GPa, and upon decompression, using methanol:ethanol:water (16:3:1) mixture (m.e.w.) as a nominally penetrating hydrostatic P-transmitting medium. No complete X-ray amorphization is observed up to the highest investigated pressure, and the original unit-cell parameters are almost completely recovered upon decompression. From 0.6 GPa, the water content is slightly higher than at ambient pressure, as a result of a moderate over-hydration. Moreover, at about 2 GPa, a significant water molecule system re-arrangement occurs, characterized by an ordering of part of the water molecules from four partially occupied sites to only two fully occupied ones. The over-hydration, but not the water ordering, is substantially irreversible upon pressure release. The Rietveld structural refinements of the powder patterns converged successfully up to 2.8 GPa; above this pressure, a phase transition to triclinic symmetry was observed and only the unit-cell parameters were refined. The comparison of the overall cell volume reductions and of the bulk modula of gismondine compressed between P_amb and 7.9 GPa in m.e.w. and in silicone oil, reveals that this is the unique zeolite with a higher compressibility in penetrating vs. non-penetrating P-transmitting media. This is ascribed to the re-organization of the water molecule system upon compression in m.e.w., which leaves a larger free volume inside the pores with respect to the phase compressed in silicone oil.

Key Words: Zeolite • gismondine • high pressure • compressibility • over-hydration • phase transition • crystal structure • synchrotron XRPD data