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

摘要

The high-pressure behavior of a synthetic mor-denite-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, β_(v0) = 1/K_(v0) = 0.040(3) GPa~(-1); K_v' = (?K_v/?P)_T = 2.0(3)], coupled with a remarkable elastic anisotropy (β_bβ_c>β_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].