In-situ synchrotron XRPD study of 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 (VALL) [2], 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 refinements, before the onset of a P-induced phase transition. XRPD experiments were performed in DAC at the BM01 beamline at ESRF with a fixed λ=0.7355 Å. Powder patterns were collected from Pamb up to 8.6 GPa and upon decompression. No complete X-ray amorphization is achieved up to the highest investigated pressure and all the features of the Pamb pattern and the unit cell parameters are well recovered upon P release. Rietveld structural refinements converged successfully up to 2.1 GPa; above this P a transition to a triclinic P-1 pseudo-rhombohedral phase was observed and consequently only the unit-cell parameters were refined. Below 2.1 GPa, a and α cell parameters slightly decrease and increase, respectively, with a resulting volume reduction of 3.6 %. The rhombohedral to triclinic phase transition is accompanied by an abrupt decrease in the unit cell parameters and in the unit cell volume (ΔV= −4.0%). Between 2.5 and 5.9 GPa, the triclinic/pseudo-rhombohedral cell parameters decrease regularly and the unit cell volume variation (ΔV=−3.0%) indicates a lower compressibility with respect to that observed before the transition. In the highest P regime (5.9-7.2 GPa), a further slope change, with an increase in compressibility, is observed. As a whole, the cell volume decrease between Pamb and 7.2 GPa is 12.6%. The elastic parameters, calculated with a second order BM-EoS, are V0 = 826 (1) Å3, K0 = 54(3) GPa and V0 = 784(2) Å3, K0 = 91(5) GPa, for the rhombohedral and triclinic phase, respectively. The deformation mechanism acting in the low-P regime can be described as a cooperative tilting of the tetrahedra belonging to the double 6-membered ring – resulting in a decrease of its thickness – accompanied by a simultaneous di-trigonalization of the two 6-rings. A similar mechanism was previously observed during compression of levyne [4]. The HP-induced cell volume contraction of NS (12.6 %) is higher than that of VALL (10.3%) in the same P range. This is congruent with the lower content in large extraframework potassium cations of NS, which contribute to sustain the porous structure in VALL.