The unusual thermal expansion of pure silica sodalite probed by in situ time-resolved synchrotron powder diffraction

The thermal behaviour of a pure silica sodalite containing trioxane was studied using the in situ synchrotron X-ray powder-diffraction technique. Rietveld refinements show that temperature-induced transformations can be schematized into three main steps: (1) The relaxation of the host-guest interactions between trioxane and the surrounding framework in the 25-200 degrees C temperature range determines a regularization in the 6-membered rings and, consequently, a process of expansion. (2) The cell volume remains constant until 380 degrees C and then starts to contract when template molecule decomposition occurs. (3) When the trioxane molecule decomposition and expulsion process is completed at above 760 degrees C, unit cell volume contraction cannot be fully justified by the negligible mass loss observed on the TG curves, thus suggesting a negative thermal expansion (NTE) process. This behaviour is unusual for sodalite-type materials which typically show positive thermal expansion upon heating, thus indicating that the presence of organic molecules instead of metal ions in the zeolite cages strongly controls the thermal expansion mechanism. (c) 2011 Elsevier Inc. All rights reserved.