Thermal response. Considering the topological sense, we expected that gis-ZIF-1 potentially exhibits a flexible nature similar to gis-zeolites. To confirm this structural flexibility, we performed temperature-dependent synchrotron SCXRD analysis, covering temperatures from 100K to 350K (Table ). As temperature increased, gis-ZIF-1 showed anisotropic thermal expansion along a and b axes while maintaining I41/a space group and gis topology. In the experiment, a and b parameters increased by 2.0 %, whereas c parameter showed a 0.2 % change (Figure ). The volume of unit cell increased by 4.4 % from 100K to 350K, and 7.6 % from 200K to 350K (Figure ). This thermal expansion was notably occurred from 200K, resembling the nonlinear expansion similar to guest-induced thermal expansion observed in MOFs. The thermal expansion coefficients were calculated from the changes in cell parameters and the cell volume (Table , Figure ). According to these calculations, gis-ZIF-1 demonstrated a colossal thermal expansion with maximum values of 690.7⨯10 -6 K -1 for a parameter and 1709.7⨯10 -6 K -1 for the cell volume at 200K. It is worth noting that the thermal expansion anisotropy observed in flexible materials arises from the molecular basis of expansion. Therefore, we initiated a detailed exploration of the gis-ZIF-1 structure to identify the precise molecular movement contributing to this anisotropic thermal expansion.