Thermoelectric properties and crystal structure of type-III clathrate compounds in the Ba–Al–Ge system

Abstract

The thermoelectric properties of type-III clathrate compounds in the Ba–Al–Ge system have been investigated as a function of Al content. The solid solubility of Al in the type-III clathrate compounds is determined to be slightly less than X=12 when expressed with the formula of Ba24AlXGe100−X. As the Al content increases, values of electrical resistivity and Seebeck coefficient increase, while that of lattice thermal conductivity decreases. The changes in electrical resistivity and Seebeck coefficient with the Al content are explained in terms of the change in the number of excess electrons upon alloying with Al, which can be described with the simple Zintl concept. The changes in lattice thermal conductivity with the Al content are explained in terms of the very low vibration frequency for the rattling motion of the Ba guest atom, which is caused by the increased size of the encapsulating cage of open dodecahedron upon alloying with Al.