Thermal conductivities of ultra-lightweight -based type-VII clathrates: an ab initio and comparative study

Abstract

Caged carbon clathrates may have profound applications in thermoelectrics due to their inherent host–guest relations. Using ab initio techniques, we investigate the electronic and phononic properties of type-VII clathrate, which is a prototype of the widely studied ternary X–B–C clathrate, and tune its properties using lithium atom filling and boron atom substitution. Upon the introduction of pairwise B substition and a Li filler, the half-filled and fully filled structures retain their semiconducting characteristics due to balanced electron counts; however, their indirect band gaps are reduced. By comparing the phonon and thermal properties of filled and empty structures, we show that B substitution and Li fillers lift the phonon degeneracies, increase the three-phonon scattering phase spaces and eventually cause tenfold reductions in the relaxation time of phonons. The Li filler-induced rattling of phonon modes and resonant scattering channels, which are validated by the peak features of phonon density of states and scattering rates, efficiently reduce the room temperature thermal conductivity for fully filled structures. Our findings indicate the efficiency of heteroatomic substitution and fillers to engineer the phonon and thermal properties of carbon-based clathrates.