Origin of low thermal conductivity in quasi-2D TlCu3Se2 with superior thermoelectric performance: An inelastic neutron scattering study

Semiconductors that simultaneously show ultralow lattice thermal conductivities (klat) and high thermoelectric figure-of-merit (zT) are rare, yet they are fundamentally intriguing and critically important for energy conversion. Recently, we have worked with TlCu3Se2, a new material being explored in thermoelectrics, and identified some of its interesting features. The material has a layered crystal structure comprised of infinite [Cu3Se2]- sheets extending along the crystallographic ab plane, and weakly-bonded Tl+ ions between two of such layers. TlCu3Se2 exhibits ultralow klat along the crystallographic c direction and the value touches the diffusion limit of thermal conductivity above 600 K, which is even lower than the glass limit for TlCu3Se2. This ultralow and anisotropic thermal conductivity in TlCu3Se2 leads to a very high thermoelectric figure of merit (zT=1.6 at 650 K) along the crystallographic c direction. Many weakly dispersive phonon modes exist in low-energy regions with very low participation ratios, mostly arising from Tl movements, which might be acting as a rattler in this system. These Tl+ vibrations drastically reduce the overall thermal conductivity of TlCu3Se2. Inelastic neutron scattering (INS) data as a function of temperature will enable us to extract phenomena on the atomic scale and help us investigate the fundamental cause for the intrinsically ultralow klat in TlCu3Se2.