Phonon-glass electron-crystal thermoelectric clathrates: Experiments and theory

Type-I clathrate compounds have attracted a great deal of interest in connection with the search for efficient thermoelectric materials. These compounds constitute networked cages consisting of nanoscale tetrakaidecahedrons (14-hedrons) and dodecahedrons (12-hedrons), in which the group-1 or -2 elements in the periodic table are encaged as so-called ``rattling'' guest atoms. It is remarkable that, although these compounds have a crystalline cubic structure, they exhibit glasslike phonon thermal conductivity over the whole temperature range depending on the states of rattling guest atoms in the tetrakaidecahedron. In addition, these compounds show unusual glasslike specific heats and terahertz-frequency phonon dynamics, providing a remarkable broad peak almost identical to those observed in amorphous materials or structural glasses, the so-called boson peak. An efficient thermoelectric effect is realized in compounds showing these glasslike characteristics. In this decade, a number of experimental works dealing with type-I clathrate compounds have been published. These are diffraction, thermal, and spectroscopic experiments in addition to those based on heat and electronic transport. These form the raw materials for this review based on advances from this decade. The subject of this review involves interesting phenomena from the viewpoint not only of physics but also of the practical problem of elaborating efficient thermoelectric materials. This review presents a survey of a wide range of experimental investigations of type-I clathrate compounds, together with a review of theoretical interpretations of the peculiar thermal and dynamic properties observed in these materials.