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.

lattice dynamics

perovskites

force constants

phonons

den- sity functional theory

anharmonicity

slow dynamics

oxides

PJ salen, Origo, Chalmers tekniska högskola
Opponent: Prof. Hannu-Pekka Komsa, Microelectronics Research Unit, University of Oulu, Finland.

Author

Petter Rosander

Chalmers, Physics, Condensed Matter and Materials Theory

Understanding Correlations in BaZrO<inf>3</inf>: Structure and Dynamics on the Nanoscale

Chemistry of Materials,;Vol. 36(2024)p. 514-523

Journal article

Tensorial Properties via the Neuroevolution Potential Framework: Fast Simulation of Infrared and Raman Spectra

Journal of Chemical Theory and Computation,;Vol. 20(2024)p. 3273-3284

Journal article

Rosander, P., Fransson, E., Österbacka, N. , Erhart, P. , and Wahnström, G. Untangling the Raman spectrum of cubic and tetragonal BaZrO3

Förstå fasövergångar i material: Ett modernt tillvägagånsätt

Fasövergångar är ett fascinerande forskningsområde inom materialfysik, där fokus ligger på hur materialens struktur förändras under olika förhållanden. Dessa förändringar kan dramatiskt påverka materialets egenskaper, och att förstå dem är avgörande för framsteg inom till exempel teknologi och materialvetenskap.

Fasövergångar sker när externa förhållanden som temperatur eller tryck ändras. Till exempel kan uppvärmning eller kompression av ett material få dess atomstruktur att omorganiseras, vilket leder till nya faser med andra egenskaper. Forskare studerar dessa övergångar genom olika experimentella tekniker, såsom röntgendiffraktion och Ramanspridning. I dessa experiment kan en ny fas upptäckas genom förändringar, som exempelvis nya toppar i spektrat. Eftersom fasövergångar är nära kopplade till hur atomerna vibrerar i materialet, är det viktigt att förstå dessa vibrationer, även kallade fononer, för att förklara beteendet hos olika faser.

I denna tes studeras två olika material som har en perovskitstruktur och med hjälp av datorsimuleringar kan vi beräkna de olika experimentella spektrumen. Utöver att beräkna de experimentella spektrumen kan vi också förklara vad de ser experimentellt.  Detta leder till en ökad förståelse av materialen och kan leda till vidareutveckling av dem.

Subject Categories

Physical Sciences

Condensed Matter Physics

Roots

Basic sciences

Infrastructure

C3SE (Chalmers Centre for Computational Science and Engineering)

Chalmers e-Commons

Areas of Advance

Materials Science

ISBN

978-91-8103-111-9

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5569

Publisher

Chalmers

PJ salen, Origo, Chalmers tekniska högskola

Opponent: Prof. Hannu-Pekka Komsa, Microelectronics Research Unit, University of Oulu, Finland.

More information

Latest update

9/24/2024