| dc.description.abstract | The modern challenge of functional materials requires us to understand the link between atomic structures and the functionality of materials. While experiments have greatly improved our understanding of these materials, there are still many properties that can not be directly interpreted experimentally. To solve this, people have come up with different computational simulation methods for research on various topics. In the investigation of functional materials, simulations can construct structural modelling at the atomic scale, demonstrate electronic properties, and reveal the dynamic behaviour of systems, all being closely related to the functionality of materials. Therefore, computational simulations are used to supplement practical experiments and support the investigation of functional materials. In my thesis, four different functional materials are investigated. All these four materials show their functional properties relevant to the dynamic structures, the motion of atoms, the rearrangement of the framework, and the responsive electronic structures. Using different simulation methods, I aim to promote the understanding of the atomic and electronic structures of materials, their dynamic behaviour, and the joint effect on the functional properties, so as to achieve the practical application of these functional materials. Ammonium sulfate shows the barocaloric effect. Combined with experiments, we show that it undergoes an order-order phase transition. Both high- and low-temperature phases are simulated using structural modelling. Through the investigation of phonon properties, including dispersion curves, phonon modes, and the response of these modes to pressure, I show that the giant entropy change is related mostly to the librational modes of the ammonium molecules. Quinuclidinium salts are a new family of colossal barocaloric materials, with promising applications in solid-state cooling. By ab initio molecular dynamics, I simulate the high- and low-temperature phases of these systems. From the analysis of the orientation of the globular molecules within the systems, I show that the configurational disorder accounts for much of the entropy change of the systems. The hybrid perovskite [(CH3)3NOH]2[KFe(CN)6] (M=Fe, Co) undergoes a multi-axial ferroelectric phase transition via an unusual bond-switching mechanism. By constructing the hypothetical intermediate centrosymmetric phase, I use the modern theory of polarisation to calculate the polarisation of the system and apportion its polarisation between contributions of the A-site cation and BX3 framework. Guanidinium copper formate also has a hybrid perovskite structure and contains Jahn-Teller active Cu ions. Learning from its inorganic analogue KCuF3, I construct a new polymorph showing another Jahn-Teller distortion mode that is not found experimentally. Symmetry-mode analysis is carried out to investigate the relation between the two phases. The stability is calculated, accounting for the fact that this hypothetical phase is not found in the experiment. By studying hypothetical structures as well as experimentally determined ones, and by investigating the shape of free energy minima as well as the minimum-energy structure itself, simulation can produce results that are not directly achievable from the experiment and help us understand the origins of functionality in materials. These studies of computational simulations can be linked to the experimental results and enrich our knowledge of the relation between the structure and the functionality, leading to a better understanding of the designed functional materials. | en_US |
