The phonon bandgap in thermoelectric SnSe: the role of dispersion interaction

Abstract

In recent years, significant progress has been made in assessing the fundamental link between the phonon band structure and lattice thermal conductivity, illuminating a path towards materials with enhanced thermal properties. In this context, SnSe has emerged as an outstanding candidate for lead-free thermoelectric (TE) devices. Due to its ultra-low thermal conductivity, SnSe has demonstrated record-high TE performance, achieving a TE figure of merit, zT, of ~ 2.6. This performance is generally accredited to strong anisotropy, ultralow thermal conductivity, and large phonon anharmonicity. Several mechanisms have been proposed to explain the low thermal conductivity, including the unstable electronic structure or softening of low-energy optical phonon modes at the Brillouin zone centre. However, these remain debated. To understand thermal conductivity, it is key to first characterize the underlying lattice dynamics, and the corresponding allowed phonon bands.

One of the distinctive features apparent in SnSe is the phonon band gap, i.e., a clear energy separation between the acoustic and optical bands. In other materials (e.g., boron arsenide), a large phonon bandgap is crucial in suppressing phonon scattering. Nevertheless, the precise size and nature of the phonon bandgap in SnSe and the mechanistic understanding of these modes have yet to be established.

In this presentation, I will show results of inelastic neutron scattering and density functional theory experiments that clarify the size and origin of the phonon bandgap in SnSe and show that the size of this gap is sensitive to the type of dispersion force considered in the calculation.