IntroductionZeolitic imidazolate framework (ZIF) material is a subset of metal-organic framework (MOF) materials with a structure similar to zeolite. Its basic structural unit is the [MN₄] tetrahedron formed by the coordination bond between transition metal center ions M (Co, Zn, etc.) and imidazole anions (Im, C₃H₃N₂^-) and their derivatives. Among them, ZIF-62 [Zn(C₃H₃N₂)_2-n(C₇H₅N₂)_n] and ZIF-4 [Zn(C₃H₃N₂)₂] crystals can melt and form glasses before decomposition, which are widely used as basic research objects for MOF vitrification. For ZIF-62 and ZIF-4 glasses, changes in metal nodes can cause changes in the local coordination structure, including bond length and tetrahedral symmetry. This could further result in non-linear variations in properties such as glass transition temperature (T_g), which is known as a mixed-metal node effect. The impact of the coexistence of mixed ligands and metal nodes in ZIF structure on thermal properties and their structural origins is not elucidated. Therefore, a further exploration of the node metal mixing effect in ZIF glasses and its dependence on the ligand composition and structure can provide some insights into the relationship between the structure and performances of multi-component ZIF glasses. It is also of great significance to clarify the mixed former effect in inorganic glass and develop novel functional MOF glasses. In this study, the effect of ligand changes on the central metal node content and T_g of ZIF-4 and ZIF-62 glasses were discussed. In addition, the structural origin about the difference in mixed-metal node effect strength [(ΔT_g)_max] between ZIF-4 glass and ZIF-62 glass was also analyzed.MethodsZIF-4/62 crystals and ZIF-4/62 melt-quenched glass (MQG) were prepared by solvothermal and melt-quenching methods, respectively. The Zn_1-xCo_x-ZIF-4 high-density amorphous phase (HDA) was prepared by differential scanning calorimetry instruments. The cobalt contents of bimetallic ZIF-4 samples were characterized by inductively coupled plasma-optical emission spectroscopy. The cobalt contents in bimetallic ZIF-62 samples were determined by ultraviolet-visible spectroscopy. The atomic structures of Zn-ZIF-4 MQG and Zn-ZIF-62 MQG were characterized by high energy synchrotron X-ray diffraction measurements. The total scattering structure factor S(Q) and pair distribution function g(r) were calculated based on the diffraction data. The in-house written code developed was used to calculate the structural parameters of MQG, i.e., the partial pair distribution functions of atom pairs, simulated total structure factor, [ZnN₄] tetrahedral order parameter (TOP), and Zn-N bond length distortion degree.Results and discussionAccording to the results of elemental analysis, the measured cobalt molar ratio (R) in the ZIF-4 and ZIF-62 crystals is lower than the initial one (x). Compared with the single-ligand ZIF-4 crystals, R in the mixed-ligand ZIF-62 crystals is closer to x. This is due to the different coordination reaction rates between different metal ions and ligands. Also, the T_g of the mixed-ligand ZIF-62 MQG is higher than that of the single-ligand ZIF-4 HDA. For instance, the T_g of Zn-ZIF-62 MQG is 31 ℃ higher than that of Zn-ZIF-4 HDA, and the T_g of Co-ZIF-62 MQG is 42 ℃ higher than that of Co-ZIF-4 HDA. These phenomena can be due to the presence of the bIm (C₇H₅N₂^-) ligand in ZIF-62 MQG. Based on the nonlinear variation of T_g with R, the mixed-metal node effect strength [(ΔT_g)_max] of ZIF-62 MQG is greater than that of ZIF-4 HDA. Specifically, the (ΔT_g)_max of ZIF-62 MQG is 7 ℃, which is greater than the (ΔT_g)_max of about 1 ℃ for ZIF-4 HDA. The comparison in total scattering structure factors S(Q) shows that the intensity of the first sharp diffraction peak in Zn-ZIF-4 MQG is slightly greater than that of Zn-ZIF-62 MQG. Moreover, from the comparison in pair distribution functions g(r), the full width at half maximum of the peaks for Zn-N₁ and Zn-C atomic connections in the coordination tetrahedron of ZIF-62 MQG are broader, compared to ZIF-4 MQG. The simulations reveal that the TOP (tetrahedral order parameter) value of ZIF-62 MQG is slightly lower than that of ZIF-4 MQG. Also, the Zn—N bond length distortion degree for ZIF-62 MQG is higher than that for ZIF-4 MQG. Based on the results, the coordination tetrahedron units of Zn-ZIF-62 MQG may exhibit a higher disordered degree, compared to Zn-ZIF-4 MQG, implying greater structural flexibility and sensitivity of ZIF-62 glass when the central Zn ion is substituted by other metal ions in the structure. This indicates that the higher (ΔT_g)_max on the T_g of ZIF-62 glass is due to the presence of mixed ligand (Im and bIm).ConclusionsBased on the solvothermal reaction mechanism of ZIF crystals, a faster reaction rate of Zn²⁺ with ligands to form complexes and a higher stability of the Zn²⁺-ligand coordination tetrahedra result in Zn²⁺ participating more rapidly in the formation of the ZIF network, thus leading to a lower R. Additionally, the stronger electron-donating property of the bIm ligand increases the coordination reaction rate of Co²⁺, leading to a higher R value in mixed-ligand ZIF-62, compared to that of single-ligand ZIF-4 crystals. According to the temperature-dependent topological constraint theory, the bIm increases both the number and strength of topological constraints in the ZIF-62 glass structure. As a result, the T_g of ZIF-62 glass is higher than that of ZIF-4 glass. The atomic arrangement in the [ZnN₄] tetrahedra of Zn-ZIF-62 MQG has a higher disordered degree, compared to that of Zn-ZIF-4 MQG. This can be attributed to the steric hindrance and stronger electron-donating ability of the bIm ligand in the Zn-ZIF-62 MQG. The higher disordered degree of tetrahedra indicates that the local coordination structure around the Zn metal nodes in Zn-ZIF-62 MQG is more susceptible to the influence of substituted Co nodes, thus enhancing the mixed-metal node effect on the T_g of Zn-ZIF-62 MQG.