Chemical and Hydrostatic Pressure in Natrolites: Pressure-Induced Hydration of an Aluminogermanate Natrolite Yongjae Lee, † Dong-Hoon Seoung, † Jianming Bai, ‡ Chi-Chang Kao, ‡ John B. Parise, § and Thomas Vogt* ,| Department of Earth System Sciences, Yonsei UniVersity, Seoul 120-749, Korea, National Synchrotron Light Source, BrookhaVen National Laboratory, Upton, New York 11973, Department of Geosciences, State UniVersity of New York, Stony Brook, New York 11794, and NanoCenter & Department of Chemistry and Biochemistry, UniVersity of South Carolina, Columbia, South Carolina 29208 ReceiVed: July 26, 2010; ReVised Manuscript ReceiVed: September 24, 2010 The ambient structure and pressure-induced structural changes of a synthetic sodium aluminogermanate with a natrolite (NAT) framework topology (Na-AlGe-NAT) were characterized by using Rietveld refinements of high-resolution synchrotron X-ray powder diffraction data at ambient and high pressures. Unlike a previously established model for Na 8 Al 8 Ge 12 O 40 · 8H 2 O based on a single-crystal study, the ambient structure of the Na-AlGe-NAT is found to adopt a monoclinic space group Cc (or Fd) with a ca. 6% expanded unit cell. The refined ambient structure of Na 8 Al 8 Ge 12 O 40 · 12H 2 O indicates an increased water content of 50%, compared to the single-crystal structure. The unit-cell volume and water-content relationships observed between the two Na-AlGe-NAT structures at ambient conditions with 8 and 12 H 2 O respectively seem to mirror the ones found under hydrostatic pressure between the Na 8 Al 8 Si 12 O 40 · 8H 2 O and the parantrolite phase Na 8 Al 8 - Si 12 O 40 · 12H 2 O. Under hydrostatic pressures mediated by a pore-penetrating alcohol and water mixture, the monoclinic Na-AlGe-NAT exhibits a gradual decrease of the unit-cell volume up to ca. 2.0 GPa, where the unit-cell volume then contracts abruptly by ca. 4.6%. This is in marked contrast to what is observed in the Na-AlSi-NAT and Na-GaSi-NAT systems, where one observes a pressure-induced hydration and volume expansion due to the auxetic nature of the frameworks. Above 2 GPa, the monoclinic phase of Na-AlGe- NAT transforms into a tetragonal structure with the unit-cell composition of Na 8 Al 8 Ge 12 O 40 · 16H 2 O, revealing pressure-induced hydration and a unit cell volume contraction. Unlike in the Na-Al,Si-paranatrolite phase, however, the sodium cations in the Na-AlGe-NAT maintain a 6-fold coordination in the monoclinic structure and only become 7-fold coordinated at higher pressures in the tetragonal structure. When comparing the pressure-induced hydration in the observed natrolite-type zeolites, Na-AlGe-NAT appears to have a nonauxetic framework and reveals the highest onset pressure for complete superhydration. Introduction Zeolites, a representative family of microporous materials, are a well-defined class of crystalline framework materials. 1 They have three-dimensional structures arising from a corner- linked framework of [TO 4 ] coordination polyhedra (T ) Si, Al, Ge, Ga, and so forth). The frameworks generally are very open and contain channels and cavities where charge-balancing cations are located together with absorbed water molecules. The cations often have a high degree of mobility, and the water molecules are readily lost and reabsorbed. At present, some 190 naturally occurring and synthetic zeolite species have been reported in the literature with the opening sizes to the pores and channels ranging from 0.2 to 2 nm. 2 Each structural and chemical feature of a zeolite contributes to the unique catalytic, molecular sieving, and selective ion-exchange properties. In addition, the flexible linkages between tetrahedra and within the secondary building units allow the structural and chemical tuning of a given zeolite as a function of composition, tem- perature, and pressure. 3-5 Recent interests in the high-pressure chemistry of zeolites stem from the discovery of pressure-induced hydration (PIH) and auxetic framework behavior in aluminosilicate natrolite and its synthetic gallosilicate analogues. 6-10 The natrolite framework is composed of T 5 O 10 (T ) Si, Al, Ge, Ga, and so forth) tetrahedral units that are connected along the c-axis forming the so-called natrolite chains. 11 This mode of linkage between the chains creates helical 8-ring channels along the c-axis with T 10 O 20 windows intersecting perpendicular to the channel. At ambient conditions, the aluminosilicate natrolite has an ordered distribution of Al and Si over the T-sites with Fdd2 (orthor- hombic) symmetry and sodium cations located along the channels and water molecules close to the T 10 O 20 windows. 12,13 When subjected to hydrostatic pressure mediated by a water- containing pressure medium, the aluminosilicate natrolite (Na 16 Al 16 Si 24 O 80 · 16H 2 O) shows an abrupt volume expansion of ca. 7% near 1 GPa, followed by a contraction of ca. 4% above 1.2 GPa. 7 This anomalous behavior is due to the successive uptake of water molecules from the pressure- transmitting media facilitated by the auxetic properties of the natrolite framework, 14 which gives rise to an ordered- paranatrolite (Na 16 Al 16 Si 24 O 80 · 24H 2 O) and a superhydrated- natrolite (Na 16 Al 16 Si 24 O 80 · 32H 2 O) phases, respectively. The ordered-paranatrolite (and natural paranatrolite) phase with * Corresponding author. E-mail: tvogt@mailbox.sc.edu. † Yonsei University. ‡ Brookhaven National Laboratory. § State University of New York. | University of South Carolina. J. Phys. Chem. C 2010, 114, 18805–18811 18805 10.1021/jp106964j  2010 American Chemical Society Published on Web 10/15/2010