Structure and Electronic Properties of a Continuous Random Network Model of an Amorphous Zeolitic Imidazolate Framework (a-ZIF)

Zeolitic imidazolate frameworks (ZIFs) are a rapidly emerging class of versatile porous material with many potential applications. Here, we report the construction of an amorphous ZIF (a-ZIF) model from a near-perfect continuous random network model of a-SiO2. The radial distribution function is in good agreement with measurements for amorphous a(T)ZIF-4 but with notable fine differences. The electronic structure and properties of the a-ZIF model are critically compared with those of three crystalline ZIF phases, ZIF-4, ZIF-zni, and ZIF-8, using density functional theory methods. We confirm the retention of the metal tetrahedral bonding coordination in a-ZIF and the nearly identical short-range ordering found in crystalline ZIFs. The considerable Zn-N bond strength plays a key role in retaining the tetrahedrally bonded network structure. The calculated optical properties of a-ZIF show a complex absorption spectrum with an ultralow refractive index n of 1.327 and a plasmon frequency of 15.810 eV.