Lattice dynamics in perovskites for green energy applications: A theoretical perspective

Licentiate thesis, 2022

Electrolyzers and fuel cells are used in green energy applications, electrolyzers split water to produce hydrogen, which can then be used in fuel cells to produce energy. Oxide perovskites have shown favorable properties for applications in this area, e.g., as electrolyte and cathode material in fuel cells and electrolyzers. The important property is the conductivity of protons, which depends sensitively on the hydrogen concentration and mobility. The concentration depends on the efficiency of the hydration reaction, which is the primary way to incorporate protons in perovskites. An example of an excellent proton conductor is acceptor doped BaZrO3. Hence, some of the most crucial material properties derive from defect properties. This thesis also explore the halide perovskites CsPbBr3, which have proven to be auspicious for photovoltaics. Insights into phase stability, phase transitions and the underlying dynamics in these materials are crucial. Thus, the understanding of microscopic properties is the cornerstone of this thesis.

In the present thesis, density functional theory is utilized to obtain training data for construction of potentials. The potentials that have been used are either force constant potentials or neural network potentials. The potential are then used to run lattice dynamics. To vastly extend the total simulation time or simply decrease the computational time, graphical processing units are also employed. Furthermore, defect models are applied to understand reaction kinetics.

More specifically, the vibrational defect thermodynamics of BaZrO3 was examined within the harmonic approximation. We also elaborate on the soft antiferrodistortive phonon mode found in this material using self-consistent phonons and molecular dynamics. This soft mode, should ultimately be the deciding factor for which structure
BaZrO3 exhibit at low temperatures Similar methods were also employed to investigate phonon dynamics in the very anharmonic perovskite, CsPbBr3. These type of insights can, e.g., further guide the development of new materials by fine-tuning of properties.