The Seebeck Coefficient of Superionic Conductors: A View from the Bond Fluctuation Model

Noble metal chalcogenides are attracting considerable interest as thermoelectric materials. These materials exhibit superionic conduction at high temperatures. This fact is certainly affecting the peculiar thermoelectric properties of these materials. However, few attentions have been paid to this fact. The Seebeck coefficient which gives the magnitude of the induced voltage in response to a temperature gradient is a fundamental quantity that characterizes the thermoelectric material. In the present report, a theoretical expression for the electronic Seebeck coefficient of superionic conductors is derived from the point of view of the Bond Fluctuation Model of ionic conduction proposed by the author. According to this model, the transport of ions is accompanied by a fluctuation in the electronic cloud surrounding the moving ion. Such a process affects the electronic properties of the materials. It is shown that the model captures the peculiar temperature dependence of the Seebeck coefficient observed experimentally.