DEHYDRATION DYNAMICS OF STILBITE USING SYNCHROTRON X-RAY-POWDER DIFFRACTION

The continuous structural transformation of the natural zeolite stilbi te .04[Fe(0.01)(3+)A1(17.33)Si(54.64)O(144)].6OH(2)O) upon dehydration has been studied using Rietveld structure analysis of temperature-res olved powder diffraction data collected with synchrotron radiation. In the initial stage of heating, the monoclinic F2/m stilbite structure (the so-called A phase) behaves as a noncollapsible framework, featuri ng only a slight framework distortion and a slight cell-volume contrac tion. At about 420 K, a first-order phase transition occurs changing t he symmetry to an orthorhombic Amma phase, whose framework is collapsi ble and shows a large cell-volume contraction with temperature. The ce ll contraction is related to the process of T-O-T bond breaking and le ads to a high-temperature stilbite phase with the same Amma space grou p and a collapsed structure similar to the previously described B phas e in stellerite and barrerite. The structural refinement indicates tha t the dynamics of bond breaking is related to the shift of the Ca cati ons in the channels to achieve optimal coordination after the release of the H2O molecules. Refined statistical occupancies of the tetrahedr al atoms involved in the bond-breaking process (T1 and T1P) are consis tent with a random rupture and re-formation of the T-O-T bonds. This i s the first experimental study of the dynamic bond breaking of T-O-T b onds in a framework structure.