Compressional behaviour of paulingite -A sub-nanosponge?

Introduction Paulingite is a rare zeolite, found in vesicles in basalt flows, with ideal chemical formula: (K,Na,Ca0.5,Ba0.5,)10(Al10Si32O84)30H2O (Z = 16). Its crystal structure was solved and refined by Gordon et al. (1966) in the space group Im3m, showing the complex framework topology of this zeolite designated with the IZA-code “PAU”. A structural re-investigation was carried out later by Lengauer et al. (1997). The tetrahedral framework topology of paulingite is characterized by a connecting double 8-ring (D8R), which links alternatively the -cage (truncated cuboctahedron) and the -cage (gmelinite-type cage). The D8R, the -cage and the -cage represent the building-block units of the PAU framework. The main voids systems of the PAU framework are represented by two parallel (and independent) sets of a three-dimensional channel systems oriented along the principal axes and shifted ½, ½, ½ against each other. Along the threefold axis of the PAU framework, a second type of a channel system exists, which is built up by the -cage and a modified form of the levyne-cage only observed in the paulingite topology (i.e., -cage) (Lengauer et al. 1997). The PAU framework type is considered as one of the most complex in the mineral world. In all the structure refinements so far reported, the Si/Al-distribution was modelled as completely disordered. A series of extra-framework sites were located. The long “free diameters” of the channel systems make this zeolite a good candidate to explore the P-induced penetration of external molecular species in response to hydrostatic compression (e.g., Gatta 2008, 2010). Experimental Methods A sample of paulingite from Vinařická hora Hill near Kladno (Czech Republic) was used for our experiments. A sample from the same locality was previously used by Lengauer et al. (1997) for their chemical and crystallographic study. Electron microprobe analysis (in wavelength dispersive mode) along with thermo-gravimetric data yielded the following chemical formula: (Ca2.57K2.28Ba1.39Na0.38)(Alll.55Si30.59O84)x 27H2O (Lengauer et al. 1997). A single-crystal of paulingite, free of defects under polarized microscope, was selected for the in-situ diffraction experiment with a diamond anvil cell (DAC). Intensity diffraction data were first collected at room-conditions with a Stoe StadiVari diffractometer with an high-brilliance Incoatec Mo Iµs X-ray-source and a Dectris Pilatus 300K pixel detector. The structure refinement was performed in the space group Im3m using the structural model of Lengauer et al. (1997) to a R1 = 0.0802 for 2477 Fo > 4(Fo) and 255 refined parameters. The same crystal was used for the high-pressure (HP) experiment performed using an ETH-type DAC. The experiment was conducted using a mixture of methanol:ethanol = 4:1 as hydrostatic P-transmitting medium, along with a few ruby chips serving as P-calibrant. Unit-cell parameters were measured between 0.0001 (crystal in the DAC with no pressure medium) and 3.3(1) GPa. Two further in-situ HP synchrotron X-ray powder diffraction experiments were performed at the X7A beamline at the national synchrotron light source (NSLS) at Brookhaven National Laboratory (BNL). A gas-proportional position-sensitive detector was used. The wavelength of the incident beam was 0.60046(1) Å as determined from a CeO2 standard. A modified Merrill–Bassett DAC was used to generate HP-conditions. Two compression experiments with two different P-fluids were performed, i.e., with silicon-oil and a mix of methanol:ethanol:water = 16:3:1. The evolution of the cell parameters with P for all three pressure-transmitting media is shown in Fig. 1. Results and Discussion The evolution of the unit-cell parameters of paulingite with P based on our experiments with different P-media show a dramatic role played by the compression-fluid on the behavior of this zeolite (Figure 1). Due to its polymeric nature, silicon-oil can be unambiguously considered as a “non-penetrating” P-medium. The compressional pattern obtained with silicon-oil describes the actual elastic behavior of paulingite (i.e., without any interference of the P-fluid). The Birch-Murnaghan equation of state truncated to the second-order was used to fit the experimental P-V data within the P-range investigated (i.e. 0.0001-2.5(1) GPa), giving the following isothermal bulk modulus: K0 = 0-1 = V0(P/V) = 18(1) GPa (0 = 0.055(3) GPa-1). Paulingite appears to be one of the softest crystalline inorganic materials reported so far. The HP-data obtained using the mix methanol:ethanol = 4:1 and methanol:ethanol:water = 16:3:1 suggest that these molecules act as “penetrating” media in response to the applied pressure. The P-induced penetration of external molecules through the cavities leads to a lower bulk compressibility of paulingite. The different compressibility of paulingite in methanol:ethanol = 4:1 and methanol:ethanol:water = 16:3:1 mix reflects the different penetrability of the media. Water is clearly the most penetrating molecule in response to the applied pressure, and so in general an hydrous medium tends to decrease significantly the compressional pattern of a porous material (Gatta 2008, 2010). Interestingly, the P-induced penetration of external molecules in paulingite structure does not lead to spectacular expansion (with a drastic discontinuity in the P-V behaviour), as observed for example in natrolite (Lee et al. 2002). The complexity of the paulingite structure did not allow to perform structure refinement at high pressure, hindering a description of the penetration mechanisms at the atomic scale. A series of further experiments are in progress in order to explore: 1) the reversibility of the P-induced penetration of aforementioned molecules and 2) the behavior of this zeolite as a “sub-nanosponge” for other small molecules in response to hydrostatic pressure. Acknowledgment GDG acknowledges the Italian Ministry of Education, MIUR-Project: “Futuro in Ricerca 2012 - ImPACT- RBFR12CLQD”. References Gatta, G.D. (2008) Does porous mean soft? On the elastic behaviour and structural evolution of zeolites under pressure. Zeitschrift für Kristallographie, 223, 160–170. Gatta, G.D. (2010) Extreme deformation mechanisms in open-framework silicates at high-pressure: Evidence of anomalous inter-tetrahedral angles. Microporous and Mesoporous Materials, 128, 78–84. Gordon, E.K., Samson, S. and Kamb, W.B. (1966). Crystal structure of the zeolite paulingite. Science, 154, 1004-1007. Lee, Y., Vogt, T., Hriljac, J.A., Parise, J.B., and Artioli, G. (2002) Pressure-Induced Volume Expansion of Zeolites in the Natrolite Family. Journal of the American Chemical Society, 124, 5466-5475. Lengauer, C.L., Giester, G., and Tillmanns, E. (1997). Mineralogical characterization of paulingite from Vinarická Hora, Czech Republic. Mineralogical Magazine, 61, 591-606.