Unraveling the effects of linker substitution on structural, electronic and optical properties of amorphous zeolitic imidazolate frameworks-62 (a-ZIF-62) glasses: a DFT study.

Zeolitic imidazolate frameworks (ZIFs), as one sub-family of metal-organic frameworks (MOFs), are a new class of glass formers. Among them, one termed as ZIF-62 [Zn(Im) 2- x (bIm) x ], has great glass-forming ability compared with other ZIFs. Yet very few reports provide us with information about this glass from the computational point of view. In order to explore and have a deeper understanding of the effects of mixing organic ligands on the fundamental properties, we systematically investigate the electronic, intrinsic bonding and optical properties of amorphous ZIF-62 (a-ZIF-62) glass with density functional theory (DFT) calculation. Here we construct six amorphous ZIF-62 (a-ZIF-62) models with the ratio of bIm/(Im + bIm) ranging from 0 to 33.5%. These models are constructed based on a near perfect continuous random network model with over one thousand atoms originated from silica glass. They all keep a short range order but lack a long range order. As the concentration of bIm increases, our results show that the internal cohesion of a-ZIF-62 increases, indicating a strong network connectivity. Especially, for the first time we reveal that an obvious mid-band emerges in the conduction band due to the addition of bIm ligands. This further leads to alternation in the dielectric function. The calculated refractive index increases with an increase in the content of bIm ligands, which agrees well with the experimental results. The present results help us have a deeper understanding of the linker ratio effects on the various properties of amorphous ZIF glass and shed light on other ways to tune the properties of amorphous ZIF glass.