The glass transition in the high-density amorphous Zn/Co-ZIF-4.

The high-density amorphous phases (HDAs) of bimetallic zeolitic imidazolate frameworks (Zn/Co-ZIF-4) were prepared. The temperature dependence of the isobaric heat capacity () of ZIF-4 HDAs was measured to determine the glass transition temperature () of HDAs. The non-linearly decreases with the molar ratio , where is Co/(Co + Zn), indicating the presence of a mixed-metal node effect. This effect arises from the non-linear increase of the degree of configurational freedom in the HDA as increases. The degree of configurational freedom is inversely correlated with the network connectivity, which is, in turn, affected by variations in the MN (M: Zn or Co; N: nitrogen) tetrahedral symmetry in the ZIF-4 HDA. Overall, this work offers valuable insights into the glass transition of metal-organic frameworks.