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: Cmcm or Cmc2(1)) 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, Pbnn or Pbn2(1). 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 [K-V0 = 25(2) GPa, beta(V0) = 1/K-V0 = 0.040(3) GPa(-1); K-V' = (partial derivative K-V/partial derivative P)(T) = 2.0(3)], coupled with a remarkable elastic anisotropy (beta(b)>>beta(c)>beta(a)), was found. Interestingly, the low-P and high-P polymorphs 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-P regime: the bulk compression is strongly accommodated by the increase of the ellipticity of the large 12-membered ring channels running along [001].