Effects of low temperature and high pressure on the structure of gobbinsite

Introduction Gobbinsite is a rare zeolite, typically found in amygdaloidal vugs of massive volcanic rocks, where it crystallizes as a hydrothermal post-magmatic product. Usually, gobbinsite forms radiating aggregates of sub-microscopic elongated crystals, often associated with phillipsite, gmelinite, natrolite and calcite. It was first described from the type locality of Co. Antrim, N. Ireland, but other occurrences were later reported from a few other localities. Its ideal formula is Na5[Al5Si11O32]•12H2O, but evidence of Ca,K-enriched form has been reported, so that the general chemical formula can be re-written as: (Na2-2x,Cax)2K2[Al6Si10O32]•12H2O. Gobbinsite has a GIS (gismondine) framework type, which can be described as a stacking of two perpendicular double crankshaft chains, the first running along [100], the second along [010]. Two secondary building units are present, 8- and 4-membered rings, with two perpendicular and interconnecting sets of channels: 8mR[100] and 8mR[010]. The ideal symmetry for the GIS framework type is tetragonal I41/amd (idealized unit cell constants: a = 9.8 Å, c = 10.2 Å), but zeolites with GIS topology often show a lower symmetry (e.g. gismondine; garronite; amicite; gobbinsite; Na-P1; Na-P2; TMA-gismondine). The lack of crystals large enough (and free of twinning and defects), forced early studies on the crystal structure of gobbinsite only by X-ray powder diffraction technique. Solution and refinement of the structure led to an orthorhombic unit cell (a = 10.108(1), b = 9.766(1) and c = 10.171(1) Å) and space group Pmn21. However, such a model showed unusually low coordination for the extra-framework cations (CN = 4 for K and 5 for Na). Later, findings of sub-millimetric crystals from Bundoora, Melbourne, Victoria, Australia, allowed us to re-investigate the gobbinsite crystal structure by X-ray single-crystal technique. The analysis of reflection conditions and statistics of distribution of normalized structure factors suggested the centrosymmetric space group Pmnb as highly likely, with unit cell a = 10.1035(15), b = 9.7819(10) and c = 10.1523(9) Å. The single-crystal structure refinement showed two extra-framework sites partially occupied by Na and Ca, respectively (though inter-sites chemical disorder could not be ruled out), five partially occupied sites for H2O molecules and a (Si,Al)-disordered distribution in the tetrahedral framework. In addition to the re-investigation of the crystal structure of gobbinsite from Bundoora (Australia), we have investigated the low-temperature (LT) and the high-pressure (HP) behavior of this zeolite by means of in-situ single-crystal X-ray diffraction. The LT experiments have been performed in order to minimize the effects of atomic thermal libration and positional disorder. The presence of many centers of motion for a single site (positional disorder) results in large and unusual anisotropic displacement parameters. LT conditions are expected to reduce such effects, leading to a clearer picture of the extra-framework configuration. Moreover, comprehension of LT-behavior could provide more information about chemical disorder in extra-framework cationic sites. The HP experiments have been performed following a series of experiments on microporous materials aimed to describe the response of this open-framework materials under hydrostatic pressure of the order of GPa. Experimental Methods Chemical analysis, by means of electron microprobe in the wavelength dispersive mode and elemental CHN analysis, yelded to the following chemical formula: (Na4.97K0.07Ca0.48)Σ5.52[Al5.62Si10.29]Σ15.91O32•11.9H2O (Z = 1) (E(%) = -6.3). Two single-crystals, free of defects under polarized microscope, has been used for the LT and HP experiments, respectively. Intensity diffraction data were collected at 293 (room temperature), 250, 200, 150 and 100 K using an Oxford Diffraction Gemini diffractometer operating at 50 kV and 40 mA with a MoKα radiation, equipped with a Ruby CCD detector, positioned at 50 mm from the sample, and an Enhance X-ray Optics graphite monochromator. Low-T data were collected with the crystal cooled by an Oxford Cryosystems 700 open-flow nitrogen gas system. The high-pressure experiment was performed using an ETH-type diamond anvil cell (DAC). The experiment was conducted using a mixture of methanol:ethanol = 4:1 as hydrostatic P-transmitting along with a few ruby chips as P-calibrant. Unit-cell parameters were measured between 0.0001 (crystal in the DAC with no pressure medium) and 4.3(1) GPa, using a list of 36 Bragg peaks centered with a KUMA KM4 point-detector diffractometer, operating at 50 kV and 40 mA, with MoKα radiation (graphite monochromator). Seven intensity data collections were performed at 0.0001, 0.8(1), 1.5(1), 2.5(1), 3.0(1), 3.7(1) and 4.3(1) GPa using an Xcalibur diffractometer equipped with a CCD. All the datasets showed reflections conditions consistent with the Pmnb space group. Structure refinements at LT and HP were performed with the SHELX-97 program. Results and Discussion a) Low Temperature Reflection conditions confirmed that the space group Pmnb is maintained within the T-range investigated. At room temperature, the extra-framework population consists of one site partially occupied by Na, one site partially occupied by Ca, and five sites partially occupied by H2O. At low-T, a partial dehydration, likely induced by the N2-flow, was observed, with a significant rearrangement of the extra-framework configuration. Low-T induced deformations of the 8- and 4-membered rings were observed, and the relationships between the main deformation mechanisms of the tetrahedral framework and the contraction along the unit-cell edges have been outlined. b) High Pressure No evidence of amorphization was observed within P-range investigated. Two changes of the elastic behavior occur: the first at 1.1 - 1.3 GPa and the second at 2.7 - 3.2 GPa. Birch-Murnaghan equations of state truncated to the second order were used to fit the experimental P-V data within the three P-ranges (i.e. 0.0001-1.1, 1.3 - 2.7 and 3.2 - 4.3 GPa), giving the following isothermal bulk moduli: 46.3(9), 52(8) and 28(6) GPa, respectively. The unit-cell compression is significantly anisotropic. In response to the applied pressure, the 8-membered ring channel perpendicular to [100] undergoes a significant increase of ellipticity, whereas the channel perpendicular to [010] undergoes a shrinking towards a circular shape at P ≥ 1.3 GPa. A partial re-organization of the H2O sites occurs between 1.1 and 1.3 GPa, and new framework deformational modes onset at P ≥ 3.2 GPa, coupled with a change in the coordination environment of the extraframework cations. If we compare the elastic behavior and the HP structural evolution of gobbinsite with those so far reported for other zeolites, we can make some generalizations: 1) The range of compressibility among this class of open-framework silicates is large, with bulk moduli ranging between 15 - 70 GPa; 2) Microporosity does not necessarily imply high compressibility – Gobbinsite has large channels, if compared to other zeolites, but its compressibility is not significantly low; 3) The flexibility observed in zeolites under hydrostatic compression is mainly governed by tilting of rigid tetrahedra around O atoms that behave as hinges within the framework; 4) Deformation mechanisms in response to applied pressure are generally dictated by the topological configuration of the framework rather than the Si/Al-distribution or the extra-framework content. The channel content governs the compressibility of the cavities. Acknowledgment GDG, PL and NR acknowledge the Italian Ministry of Education, MIUR-Project: “Futuro in Ricerca 2012 - ImPACT- RBFR12CLQD”.