HP-INDUCED PHASE TRANSITION IN CHABAZITE FROM NOVA SCOTIA

The response to high pressure (HP) of a natural chabazite from Nova Scotia (Canada) (s.g. R-3mR [1]) was investigated by means of in situ synchrotron XRPD and silicone oil as non-penetrating P-transmitting medium, in the frame of a wider project aimed at understanding the role of the framework/extraframework content on the compressibility of CHA-type porous materials. Previous studies performed on natural chabazite from Vallerano [2] (VALL), SAPO-34 [2] and ALPO-34 [3] provided important information on their HP behavior, but no complete structural description of the response of these microporous phases to compression was possible. In this work, the P-induced deformation mechanism of chabazite from Nova Scotia (NS) was followed by full profile Rietveld refinement, before the onset of a P-induced phase transition. XRPD experiments were performed in DAC at the BM01 beamline at ESRF (Grenoble, France) with a fixed λ=0.7355 Å. Powder patterns were collected from Pamb up to 8.6 GPa and upon decompression. Data analysis was performed only up to 7.2 GPa due to the low quality of the highest pressure patterns. Unit-cell parameters were refined in the R-3mR s.g. up to 2.1 GPa using Rietveld method. Above this pressure, a phase transition from rhombohedral to triclinic P-1 (pseudo-rhombohedral) symmetry was observed and hence only the unit-cell parameters were obtained by using Le Bail method. In correspondence to the phase transition, an abrupt decrease of most of the cell parameters and of the cell volume was observed. Above the transition - between 2.5 GPa and 5.9 GPa - the triclinic/pseudo-rhombohedral cell parameters decrease regularly, with a decrease in compressibility with respect to the trend observed before the transition. A further compressibility increase occurs in the highest P regime, between 5.9 and 7.2 GPa. As a whole, the cell volume decrease in the investigate P-range is 12.6%. Notwithstanding the abrupt and large cell volume decrease accompanying the phase transition, this is reversible (with a significant hysteresis effect) upon decompression. The results of the complete structural refinements performed up to 1.1 GPa indicated that: i) the double-six-membered ring present in chabazite framework undergoes a flattening, as a results of a HP-induced cooperative tetrahedral anti-rotation; ii) the 8–membered rings limiting the channel apertures become more circular; iii) the extraframework content does not show relevant modifications under pressure - as concerns the site occupancies - while some significant changes are observed in the bond distances. Comparing the elastic behavior and the deformation mechanism of NS with those of VALL, a higher HP-induced cell volume contraction is registered in NS in the same P range. This is congruent with the lower content in large extraframework potassium cations of NS, which contributes to sustain the porous structure in VALL.