Ab Initio Study of Amorphous Zeolitic Imidazolate Framework (A-ZIF) and Pyrophosphate crystals

Abstract

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 aTZIF-4 but with notable fine differences. We confirm the retention of the metal tetrahedral bonding coordination in a-ZIF and the nearly identical short range ordering found in crystalline ZIFs (ZIF-4, ZIF-zni, and ZIF-8). Zn-N bond strength plays a key role in retaining the tetrahedrally bonded network structure. In addition, we studied deformation behavior of this unique a-ZIF model by simulating step-wise compression and expansion with strains between -0.389 and +0.376. An insulator-to-metal transition is observed at 51 GPa leading to a multicomponent light amorphous alloy of only 3.68 g/(cm)³. A high-density amorphous-to-amorphous phase transition is observed due to the sudden formation of N-N bond pairs. Systematic expansion of a-ZIF retains the framework softness until it fractures at high strain. Based on the expansion data, we propose an empirical formula for super-soft materials, which is in line with available experimental data. The electronic structure, mechanical and optical properties of five pyrophosphate crystals with very complex structures are studied. Existing structures from single-crystal X-ray diffraction shows short O-H bonds, which are rectified by density functional theory (DFT) calculations. The complex interplay of the minor differences in specific local structures and compositions are discussed by dividing the pyrophosphate crystals into three structural units. H₂P₂O₇ is the most important and dominating unit in pyrophosphates. The other two are the influential cationic group with metals and water molecules. The strongest P-O bond in P₂O₅ is the strongest bond for crystal cohesion, but O-H, N-H bonds also play an important part. Different type of bonding between O and H atoms such as O-H, hydrogen bonding and bridging bonds are present. Metallic cations such as Mg, Zn and Cu form octahedral bonding with O. Metallic elements can influence the structure and bonding to certain extent. The two Cu-containing phosphates show the presence of narrow metallic bands near the valence band edge. All these complex bonding affects their physical properties indicating that the fundamental understanding remains an open question.