In situ synchrotron studies of open-framework silicates at non-ambient temperature and pressure

The combined use of synchrotron X-ray diffraction (XRD) techniques and devices for in situ studies at non-ambient temperature and/or pressure allowed a deep investigation of the behavior of open-framework silicates at these conditions. Displacive phase transitions are common mechanisms adopted by framework compounds to accommodate the bulk expansion or contraction, whenever structural distortion is no more possible or energetically efficient. The zeolite mordenite, for example, crystallizes, at ambient condition, in the Cmc21 space group and undergoes a P-induced transition to a primitive polymorph. In situ single-crystal synchrotron XRD allowed to identify the space group symmetry (Pbn21) of the high-P phase and solve its framework structure, allowing to describe the deformation mechanisms triggered by the phase transition at the atomic scale. In the case of minerals, the fundamental thermo-elastic parameters and their relationship with the crystal structure can be accurately determined. Scapolites are common metamorphic minerals able to accommodate volatiles down to the lower crust, which members represent a complex non-binary solid solution. Modelling the role played by the crystal chemistry on the scapolites behavior is possible by investigating the response of the solid-solution members to T and P variations. Our group recently investigated the behavior of an intermediate scapolite (with anomalous I4/m symmetry) by in situ XRD studies at high-P (ambient-T), high-T (ambient-P) and combined high-T and P, at synchrotron facilities, providing a comprehensive characterization of the elastic and structural response, as well as of a pressure-controlled phase transition to a triclinic polymorph (at ~ 9-10 GPa) observed at 25 and 650 °C. In situ synchrotron studies on framework silicates at variable P/T also allows a better understanding of phenomena, which may be exploited in materials science and technological applications, in particular promoting crystal-fluid interactions at extreme conditions. MFI-zeolites, for example, can be adopted as catalysts in the methanol-to-olefin conversion and pressure may be adopted as a tool to improve the process efficiency, by promoting a larger loading of methanol molecules into the zeolites structural pores. In situ high-P powder XRD experiments on all-silica (silicalite) and slightly cation-exchanged MFI zeolites, using non-penetrating silicone oil and penetrating methanol as P-fluids, showed a higher efficiency in methanol adsorption by pure silicalite in the lower pressure regime and, conversely, a higher methanol intrusion in cation-exchanged zeolites at P > 0.5 GPa.