High-pressure behavior of synthetic mordenite-Na: an in situ single-crystal synchrotron X-ray diffraction study

The high-pressure behavior of a synthetic mordenite-Na (space group: Cmcmor Cmc21) was studied by in situ single-crystal synchrotron X-ray diffraction with a diamond anvil cell up to 9.22(7) GPa. A phase transition, likely displacive in character, occurred between 1.68(7) and 2.70(8) GPa, from a C-centered to a primitive space group: possibly Pbnm, Pbnnor Pbn21. Fitting of the experimental data with III-BM equations of state allowed to describe the elastic behavior of the high-pressure polymorph with a primitive lattice. A very high volume compressibility [KV0= 25(2) GPa, βV0= 1/KV0= 0.040(3) GPa–1; KV′= (?KV/?P)T= 2.0(3)], coupled with a remarkable elastic anisotropy (βb>>βc>βa), was found. Interestingly, the low-Pand high-Ppolymorphs show the same anisotropic compressional scheme. A structure collapse was not observed up to 9.22(7) GPa, even though a strong decrease of the number of observed reflections at the highest pressures suggests an impending amorphization. The structure refinements performed at room-P, 0.98(2) and 1.68(7) GPa allowed to describe, at a first approximation, the mechanisms that govern the framework deformation in the low-Pregime: the bulk compression is strongly accommodated by the increase of the ellipticity of the large 12-membered ring channels running along [001].