Equilibrium Born-Oppenheimer molecular-dynamics exploration of the lattice thermal conductivity of silicon clathrates

Abstract

Thermal conductivity plays a critical rôle in determining the performance of a thermoelectric material. However, the computation of thermal conductivity from first principles is a challenging and laborious process, with pitfalls galore. Here, we present results on the calculation of the lattice thermal conductivities of silicon clathrates using a very efficient scheme based on the Einstein relationship for the energy moment, sampled from ab initio Born-Oppenheimer molecular dynamics employing linear-scaling Density Functional Theory. Both sI and sII silicon clathrates were studied using systems consisting of up to thousands of atoms. Instead of more glass-like behaviour reported for an empty sII clathrate, the present accurate first-principles calculations predict a thermal-conductivity temperature dependence more in keeping with that of a normal crystal for both clathrates polymorphs, essentially in agreement with a previous theoretical study on sI clathrate. Over the temperature range from 50 to 600K at ambient pressure, it is found that the sI-structure thermal conductivity is somewhat lower than sII, due probably to the greater proportion of larger tetrakaidecahedral 51262 cavities (vis-à-vis smaller dodecahedral 512 cages) therein leading to more substantial dissipation of heat-carrying acoustic phonons. However, the absolute conductivity of sII clathrate is itself limited due to the larger number of atoms per unit cell (136 versus 46 for sI) leading to significant zone-edge damping of acoustic phonons.