Giant reduction in thermal conductivity of extended type-I silicon clathrates and prominent thermal effect of 6d guest Wyckoff positions

Abstract

Clathrates, which consist of a large polyhedral cage framework encapsulating guest atoms, exhibit exceptional properties such as those of high-performance thermoelectrics due to the weak electrostatic interactions between the guest atoms and the host. In this study, by performing Green–Kubo equilibrium molecular dynamics simulation, we calculate the lattice thermal conductivity (TC) of the extended type-I clathrates (Si_x and Ba₈Si_x, x = 46, 230 and 644). The extension of the cages has a huge reduction effect on the TC of clathrates, e.g. the TC of Si₆₄₄ (i.e. extended type-I clathrates) is five-fold lower than that of Si₄₆ (i.e. basic type-I clathrates). By examining the phonon behaviors, we identify that the mechanism originates from the enhanced localization of phonons in the extended cages. Furthermore, we discover that the different Wyckoff positions of the guest atoms also impose a tremendously different effect on the TC of clathrates, which has never been reported before. By introducing an isotope of barium atoms, it is interesting to find that the model with this isotope at the 6d Wyckoff position expresses much lower TC than that of the isotope at the 2a position (with a reduction of 30–40%). Combined with the phonon spectral energy density analysis, it is observed that the introduction of the isotope at the 6d position will lead to the reduction of the phonon group velocities of low-frequency phonons and the presence of new phonon scattering channels and anisotropic anharmonicity. These results unambiguously demonstrate that the isotope at the 6d Wyckoff position in type-I clathrates has much stronger effect on TC reduction than that at the 2a position and it will play more important role in the application of clathrate-based nanomaterials in the field of thermoelectricity. Our results indicate that the extension of the cages and the increased isotope concentration at the 6d Wyckoff position are two effective methods to reduce the TC of type-I clathrates and improve the thermoelectric performance. Our investigation also provides an important guidance to further construct new types of silicon clathrates for the relevant thermoelectric applications.