Effects of guest atomic species on the lattice thermal conductivity of type-I silicon clathrate studied via classical molecular dynamics

Abstract

The effects of guest atomic species in Si clathrates on the lattice thermal conductivity were studied using classical molecular dynamics calculations. The interaction between a host atom and a guest atom was described by the Morse potential function while that between host atoms was described by the Tersoff potential. The parameters of the potentials were newly determined for this study such that the potential curves obtained from first-principles calculations for the insertion of a guest atom into a Si cage were successfully reproduced. The lattice thermal conductivities were calculated by using the Green-Kubo method. The experimental lattice thermal conductivity of Ba8Ga16Si30 can be successfully reproduced using the method. As a result, the lattice thermal conductivities of type-I Si clathrates, M8Si46 (M = Na, Mg, K, Ca Rb, Sr, Cs, or Ba), were obtained. It is found that the lattice thermal conductivities of M8Si46, where M is IIA elements (i.e., M = Mg, Ca, Sr, or Ba) tend to be lower than those of M8Si46, where M is IA elements (i.e., M = Na, K, Rb, or Cs). Those of mM8Si46, where m was artificially modified atomic weight were also obtained. The obtained lattice thermal conductivity can be regarded as a function of a characteristic frequency, fc. That indicates minimum values around fc=2-4 THz, which corresponds to the center of the frequencies of the transverse acoustic phonon modes associated with Si cages.