Lattice Dynamics And Thermal Transport In Part- Crystalline Materials

As a kind of environmental friendly materials, thermoelectric compounds can convert heat to electrical power directly, by utilizing the Seebeck and Peltier effects. Good thermoelectric materials always have excellent electrical transport properties and an extremely low thermal conductivity. Due to the relatively small contribution to the total thermal conductivity, the electrical part, which is dominated by electrical conductivity, can be neglected. Reducing lattice thermal conductivity therefore can lead to a noticeable enhancement in thermoelectric performance. Traditional understanding in thermal transport and lattice thermal conductivity is mainly based on phonon-phonon non-linear interaction, defects, and microstructure scattering in the framework of perturbative theory. Recent work has gone beyond the conventional knowledge of crystalline state, and extremely low lattice thermal conductivities were often observed as in typical thermoelectric compounds, such as filled skutterudites, Cu-based compounds (Cu3SbSe3 and Cu2Se).In this work, a concept of part-crystalline part-liquid, or even part-crystalline part-amorphous state was proposed to describe materials with chemical bond hierarchy. In such a state, material could intrinsically manifest the coexistence of rigid crystalline framework and soften fluctuating substructure. A rattling-like thermal damping can effectively describe the thermal, damping caused by the part-amorphous or part-liquid substructure. Cu3SbSe4 and CuSbSe2 compounds are both in the crystalline state, while Cu3SbSe3 is in the "part-crystalline" state. The diverse lattice dynamics and corresponding harmonic and anharmonic properties of these ternary Cu-Sb-Se compounds should be attributed to the intrinsic structural characteristics. This work also reveals that the room-temperature Cu2Se should have a group of structure units, which forms a structure family. Furthermore, the room-temperature Cu2Se contains a pseudo-fcc Se framework (part-ordering) and diverse configurations of Cu substructure (part-disordering), leading Cu2Se to a part-crystalline state. As a result of substructure fluctuating, Cu2Se also displays a polymorphous phase transition, in which all the structure units directly transform to the cubic phase without any energy barriers.Due to the extremely low lattice thermal conductivity, part-crystalline materials reveal their great potential in developing and designing high-performance thermoelectric compounds. Moreover, this material state would theoretically render an integrated understanding in crystalline, amorphous and liquid states.