CRYSTAL STRUCTURES AND PHASE TRANSITIONS OF QUARTZ-TYPE ZN- AND CD- MOFS AT HIGH TEMPERATURE

Hydrothermal synthesis of [M(2-Et-Im)2] (M = Cd, Zn and 2-Et-Im = 2-ethylimidazolate) brings to the formation of low-quartz analogue phases, with space group P3121, typically as fine powders. In such metal-organic frameworks, tetrahedral M2+ coordination centres and bent anionic ligand bridges act as the tetrahedral Si4+ cations and O2- bridges in pure SiO2 quartz. In general, this class of compounds are thermally and chemically stable and a number of such phases (commonly known as Zeolitic Imidazolate Frameworks or ZIFs) have open porous structures with prospects for gas separation [1]. Differential Scanning Calorimetry (DSC) and X-ray powder diffraction at variable temperature (VT-XRPD) indicate that the two compounds used in this work undergo phase transitions at temperatures of around 300oC for Zn and 400oC for Cd. Since the structure of the related polymer [Fe(2-Me-Im)2] was found to be in the high quartz P6422 space group [2], our assumption was that the phase transitions might be from a low- to a high-quartz structure. Improved crystal growth of [M(2-Et-Im)2] was found using 3:1 acetonitrile:water mixtures at 110oC for several days, giving specimens of up to 0.5 mm in size with variable racemic twinning. Therefore an in situ high-temperature single-crystal diffraction (HT-SCXRD) study has been undertaken. Preliminary results indicate a continuous phase transition, with no change of space group and a small deviation in the thermal evolution of unit-cell parameters and volume. In this case the crystal is mounted using glue for HT. When instead the crystal is mounted in a quartz vial and kept still with the use of quartz wool to avoid mechanical stress on its surfaces, a different behaviour is noted, and a sudden change in the evolution of unit-cell parameters occurs at the transition. Work is in progress to characterize the phase transition under these conditions. New in situ HT-SCXRD measurements are in hand where the crystal is kept under vacuum.