title: Non-framework cation migration and irreversible pressure-induced hydration in a zeolite.
abstract: 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-framework metal ions, thus emphasizing that they can clearly have an important role in mediating the overall properties of zeolites.
authors.affiliation: Physics Department, Brookhaven National Laboratory, Upton, New York 11973, USA.
authors.forename: Yongjae
authors.initials: Y
authors.lastname: Lee
authors.forename: Thomas
authors.initials: T
authors.lastname: Vogt
authors.forename: Joseph A
authors.initials: JA
authors.lastname: Hriljac
authors.forename: John B
authors.initials: JB
authors.lastname: Parise
authors.forename: Jonathan C
authors.initials: JC
authors.lastname: Hanson
authors.forename: Sun Jin
authors.initials: SJ
authors.lastname: Kim