Clathrate Thermoelectrics

Abstract

The type I and II clathrate hydrate structure can be thought of as a derivative of the four coordinated diamond lattice structure. In the Ge diamond lattice there is not enough space to hold Sr atoms between the Ge atoms, for example. The presence of these “guests” induces a change in the Ge clathrate to a more open structure: the clathrate structure. These types of “open structured” compounds have unique properties that are of interest for thermoelectric applications.^{1, 2}The fact that clathrate compounds can be synthesized to possess glass-like lattice thermal conductivity and the ability to vary the electronic properties by changing the doping level in semiconducting variants, along with relatively good electronic properties, indicates that this system is a Phonon-Glass Electron Crystal (PGEC) system³and therefore of interest for thermoelectric applications. The ideal PGEC system would possess poor thermal properties (such as that for amorphous materials) while also possessing good electrical properties (as in perfect crystals). From the definition of the dimensionless figure of merit (ZT = S ² T/ρκwhere Sis the Seebeck coefficient, Tis the absolute temperature, ρis the resistivity and κthe thermal conductivity) it is clear that a PGEC system would possess optimal thermoelectric properties. The key however is to replace the traditional alloy phonon scattering, which predominantly scatters the highest frequency phonons, by a much lower frequency resonance or disorder type scattering. This is the case in these materials, due to their unique crystal structure, and is why these materials have a low thermal conductivity. In these materials certain aspects of investigations of “atomic engineering” on the nanoscale also presents itself through the role of the cage-like structures and the ability to fill the atomic cages with various types of atoms. Their crystal structure is one of the most conspicuous aspects of these compounds and directly determines much of their interesting and unique properties, including their thermoelectric properties, as will be described in detail below.