Lattice Dynamics in materials for energy applications

Lattice dynamics are of the utmost importance in understanding many modern materials. They enable us to probe the specific details of a material’s chemistry and its ordering, and are vital to understanding many technologically relevant phenomena. Unfortunately for anything more than the simplest system understanding how the dynamics relate back to the properties becomes extremely challenging. This difficulty can be overcome by combining the latest experimental techniques at international x-ray and neutron scattering facilities with first-principles calculations. The experiments allow us to validate the calculations and the calculations can then be related back to an understanding of the material’s properties. In this thesis two different problems are investigated. The first is a study of the vibrational spectrum of a thermoelectric material NaxCoO2. This system has cage-like structures with atoms inside the cage that can rattle. In similar materials this rattling has been postulated to suppress the thermal conductivity by phonon scattering. We find that in fact there is no significant phonon scattering and instead the suppression is due to a reduction in the phonon velocities. The effect of changing the cage-like structures and doping the rattling ion is also investigated. The second system studied is the alkaline doped iron selenide superconductors. Spectroscopic studies have shown that these exhibit a symmetry breaking phase transition on cooling but its origin is unknown. We show that this arises from a subtle ferrimagnetic transition related to the localisation of charge on certain iron sites. It is this charge localisation which is responsible for the symmetry breaking and by relating these results to the literature the case is put forward that the ferrimagnetism is also related to the origin of superconductivity in this system.