Structural Dynamics, Anharmonicity and Materials Modelling

Activity: Talk or presentationInvited talk


The physical properties of solids are intimately linked to their structural dynamics. The lattice vibrations (phonon modes) in crystals give rise to the natural thermal motion seen in X-ray structures, the vibrational spectra we measure routinely in the laboratory, and the heat transport in semiconductors and insulators. Thermal occupation of the phonon modes makes an important contribution to the finite-temperature free energy, which in turn drives thermal expansion and indirectly affects other properties such as the electronic structure.

The theory of lattice dynamics provides a framework for including dynamical effects in theoretical models, and modern electronic-structure techniques allow the lattice dynamics of complex materials to be modelled with high accuracy. Calculations within the harmonic approximation (HA) are fast becoming routine in materials modelling. However, many contemporary research areas require methods that can go beyond the HA and explore the anharmonic phenomena that underpin many important and unusual properties.

In this talk, I will discuss two such areas: modelling thermal transport and studying systems with so-called “soft-mode” phonon dynamics.

The ability to model phonon lifetimes and thermal transport allows materials with unusual thermal transport to be examined in microscopic detail. In conjunction with high-resolution neutron-scattering measurements, we have studied the phonon lifetimes in the archetypal hybrid halide perovskite (CH3NH3)PbI3 (MAPbI3). The calculations provide a direct link between the dynamics of the molecular cation, strong phonon scattering and the measured picosecond vibrational lifetimes. The ultra-low thermal conductivity in hybrid materials has recently been implicated in their low carrier recombination rates, and these findings thus have important consequences for their photovoltaic applications.

A number of important materials display “soft-mode” phonon dynamics, where one or more high-symmetry crystal phases are averages over distorted local minima separated by a shallow energy barrier. This leads to large deviations from the average structure and makes theoretical study of these materials challenging. We have developed a simple theoretical model for treating soft modes which allows us to map the potential-energy surface, calculate the vibrational wavefunctions and quantify the effect of the soft modes on material properties. While still under development, we have successfully used this approach to study the the phase transition sequence and electron-phonon coupling in MAPbI3 and the spontaneous polarisation in ZnO, all with encouraging results.
Period27 Feb 2019
Degree of RecognitionLocal