Phonon anharmonicity in energy materials

Description

Phonon anharmonicity is an increasingly common theme in modern materials science. Understanding the interplay between anharmonic structural dynamics and material properties is an active research area, and one that could ultimately lead to new design paradigms for functional materials. Accurate first-principles techniques such as DFT, together with modern computing capabilities, have made it possible to use first-principles lattice-dynamics calculations to explore anharmonicity at the microscopic level and make quantitative predictions of e.g. phonon lifetimes and lattice thermal conductivity. This talk will discuss two examples where modelling has played a leading role in linking structural dynamics to key properties.

Orthprhombic SnSe was recently shown to have record-breaking bulk thermoelectric efficiency, due in part to its “ultralow” lattice thermal conductivity. Calculations reveal the high-temperature Cmcm phase to be a crystallographic average over lower-symmetry Pnma minima, resulting in an anharmonic double-well potential and substantial softening of the phonon frequencies. By mapping the potential-energy surface along the soft modes, we have characterized the phase transition and developed an approximate renormalization scheme to quantify the effect of the soft modes on the thermal transport.

Hybrid halide perovskites such as the archetypal methylammonium lead iodide (MAPbI3) have had a huge impact on photovoltaics research, and have raised questions on the fundamental relationship between their structural dynamics and physical properties. In conjunction with experimental measurements of the phonon frequencies and linewidths, we have applied lattice-dynamics calculations to examine the interactions between the organic cation and the perovskite cage in MAPbI3, and to elucidate the microscopic origin of its picosecond phonon lifetimes. Our results highlight the unique structural dynamics of this class of materials compared to conventional semiconductors, and have important implications for carrier cooling and charge transport.

Period	5 Sept 2018
Event title	3rd UK Materials Chemistry Consortium (MCC) Conference
Event type	Conference
Location	Lincoln, United KingdomShow on map
Degree of Recognition	National