Dehydration dynamics of analcime by in situ synchrotron powder diffraction

The continuous structural transformation of tetragonal analcime (Na15.87Al1 5.20Si32.64 O-96. 16.3H(2)O) upon dehydration was studied, using Rietveld s tructure analysis of temperature-resolved powder diffraction data collected using synchrotron radiation. The variation of the a-e axis length differen ce and normalized intensity of the (200) reflection as a function of temper ature suggest that tetragonal analcime evolves toward a cubic structure at high temperature. The removal of water was accompanied by a spreading of th e initial Na sites into many positions bonded to the framework O atoms. The migration of H2O molecules through the [111] channels during dehydration c aused the six-member ring apertures to open as widely as possible: this was accompanied by a twisting of the tetragonal prism, constituting the analci me framework, which led to an opposite tilting of tetrahedra connecting the prisms. These modifications induced by water diffusion an not energeticall y favored because they would increase the elastic energy of the system, and require a substantial thermal activation energy. The analcime framework re ached a maximum distortion at about 650 K, the temperature of complete wate r loss, then underwent a relaxation process during which the T-O-T angles w ere restored to the starting value. The relative variation of cell volume a ssociated with the opening of wide six-member ring channels during water mi gration, and then due to the framework relaxation process after complete de hydration, provides an explanation of the "negative thermal expansion" (i.e ., volume contraction) effect in dehydrated analcime, which is complementar y to that based on the Rigid Unit Modes theory.