ELASTIC BEHAVIOR AND HIGH PRESSURE-INDUCED PHASE TRANSITION IN CHABAZITE: NEW DATA FROM A NATURAL SAMPLE FROM NOVA SCOTIA

The high pressure behavior of a natural chabazite from Nova Scotia (Canada) [(Ca1.32K0.45Na0.13Sr0.10) [Si8.55Al3.45O24]center dot 11.30 H2O, s.g. R-3m, a = 9.38749(8) and alpha = 94.379(1)] was investigated by means of in situ synchrotron X-ray powder diffraction and silicone oil as non-penetrating P-transmitting medium, in the P-range from P-amb to 7.2 GPa. Above 2.1 GPa, a phase transition from rhombohedral to triclinic (pseudo-rhombohedral) symmetry is observed. The phase transition is accompanied by an abrupt decrease of most of the cell parameters and of the cell volume. Above the transition - between 2.5 and 5.9 GPa - the triclinic/pseudo-rhombohedral cell parameters decrease regularly, with a compressibility decrease relative to the trend observed before the transition. A further increase in compressibility occurs in the highest P regime, between 5.9 and 7.2 GPa. Overall, the cell volume decrease in the investigated P-range is 12.6%. Notwithstanding the abrupt and marked cell volume decrease accompanying the phase transition, this is reversible upon decompression. The refined elastic parameters were V-0 = 826(1) angstrom(3), K-0 = 54(3) GPa and V-0 = 784(2) angstrom(3), K-0 = 91(5) GPa, for the rhombohedral and triclinic phase, respectively. 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 flattening as a result of a HP-induced cooperative tetrahedral anti-rotation; (ii) the 8-membered rings, limiting the channel apertures, become more circular and their crystallographic free area decreases; (iii) the extraframework content does not exhibit relevant modifications under pressure - as concerns the site occupancies - while some significant changes are observed in the bond distances. The elastic behavior and deformation mechanism of chabazite from Nova Scotia are compared with those of another chabazite sample from Vallerano and with those of other porous materials belonging to the 6-ring zeolite family.