Computer simulations of solids have become increasingly powerful and predictive, particularly, those based on first-principles approaches such as density functional theory (DFT). Lattice dynamics has also become an important theme related to materials science and is essential for understanding the thermal properties of crystalline solids at finite temperatures. This thesis focuses on understanding the nature of collective atomic vibrations (phonons) in solids and their role in structural phase transitions. First-principles lattice dynamics approaches - within the harmonic and quasi-harmonic approximations - are applied to describe phase transformations in the metal halide perovskite CsSnI3 and the nature of the cubic to rhombohedral ferroelectric distortion in the semiconductor GeTe. The aim is therefore to demonstrate the advantages and shortcomings of quasi-harmonic approximations and to benchmark this method on these two challenging systems. Further approaches were applied to GeTe in order to evaluate the effect of the different levels of physical complexity regarding the nature of the transition, e.g. quasi-particle self-consistent GW (QSGW). In addition to the role of temperature, we also probe the effect of an external bias on phase stability taking the case of AB and AA bilayer graphene under an applied voltage.
|Date of Award||22 Nov 2018|
|Supervisor||Aron Walsh (Supervisor), Paul Raithby (Supervisor) & Steve Parker (Supervisor)|