Amorf malzemelerin modellenmesi ve incelenmesi

The aim of this PhD dissertation is to investigate the behavior of different MOFs under hydrostatic and uniaxial stresses by using ab-inito molecular dynamics simulations (AIMD). The results obtained from computations are reported in three main chapters. In the first part, ab initio simulations within a generalized gradient approximation (GGA) were carried out to investigate the response of MOF-5 to high pressure. Similar to the previous experimental findings, a pressure-induced amorphization (PIA) was observed at 2 GPa through the simulations. The phase transformation was an irreversible first order transition and accompanied by a volume collapse around 68%. Remarkably, the transition arose from local distortions and contrary to previous suggestions, did not involve any bond breaking or formation. Additionally, a drastic band gap closure was perceived for the amorphous state. For the second part of this project, AIMD simulations were performed to probe the high-pressure behavior of ZIF-8 over wide pressure-range. Under compression, the enormous distortions in the ZnN4 tetrahedral units led to a crystal-to-amorphous phase transition at around 3 GPa. During the amorphization process, the Zn-N coordination was retained. No other phase change but a possible fracture of the system was proposed above 10 GPa. When the applied pressure was released just before the amorphization, the rotations of imidazolate linkers (swing effect) caused an isostructural crystal-to-crystal phase transition. In the tensile regime, no phase transition was perceived up to −2.75 GPa at which point the structural failure was observed. In the last part of this research project, the phase transitions of ZIF polymorphs (ZIF-1 to ZIF-3) under pressure were comprehensively simulated. ZIF-1 showed some consecutive crystal-crystal and crystal-amorphous phase transitions between -2 GPa (tension) and 10 GPa (compression). On the other hand, ZIF-2 and ZIF-3 presented similar pressure-volume relation in both tension and compression regions. In compression region, a rapid crystal-amorphous at relatively lower compression regime and most likely an amorphous-amorphous transition were explored whereas the structural failure was observed at around -3 GPa for all ZIFs.