Phase transitions, scattering and soft dynamics in perovskites

Doctoral thesis, 2024

The study of phase transitions is a critical area of research in condensed matter physics.
In the context of this thesis, phase transitions are related to the change of the underlying atomic structure of the material.
A phase transition can, e.g., be driven by increasing pressure or temperature and can directly influence the material’s properties.
Experimentally, the phase transition can be studied via scattering experiments where the phase transition can be detected via, e.g., a new Bragg peak in X-ray diffraction or the
appearance of new peaks in Raman scattering.
Moreover, the scattering experiments are intimately linked to the vibrational properties of materials. Therefore, it is of high importance to understand the vibrational properties of a material. In recent years, there has been a surge in efforts to improve the efficiency and accuracy of simulations related to vibrational properties.
Consequently, we can now develop interatomic potentials that are both fast to evaluate and highly accurate.
These advanced techniques, open up the possibility to study scattering experiments over a wide range of temperatures and pressures and for large systems and long times. In this thesis, density functional theory is employed to generate training data for the construction of such potentials.
The potentials are either force constant potentials or neural network potentials.
The developed models are subsequently used for lattice dynamics and molecular dynamics simulations.
To significantly extend the total simulation time of the molecular dynamics or to reduce the computational time, graphical processing units are utilized.
This thesis then employs such interatomic potentials to, e.g., examine the soft antiferrodistortive phonon mode in barium zirconate using self-consistent phonons and molecular dynamics.
This soft mode is expected to be the determining factor for which structure BaZrO3 adopts at low temperatures.
Moreover, due to the low frequency of this mode, there has been debate about the possibility of emerging local structures associated with the mode.
To explore this, we investigated the local dynamics of the octahedral tilt and conducted simulations of the Raman scattering, neutron scattering and electron diffraction.
Similar methods were employed to study the phonon dynamics in the highly anharmonic perovskite, CsPbBr3.