  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 nano-scale
tetrakaidecahedrons (14 hedrons) and dodecahedrons (12 hedrons), in which the
group 1 or 2 elements in the periodic table are encaged as the so-called
rattling guest atom. It is remarkable that, though these compounds have
crystalline cubic-structure, they exhibit glass-like 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 glass-like specific heats and THz-frequency phonon dynamics,
providing a remarkable broad peak almost identical to those observed in
topologically disordered amorphous materials or structural glasses, the
so-called Boson peak. An efficient thermoelectric effect is realized in
compounds showing these glass-like characteristics. This decade, a number of
experimental works dealing with type-I clathrate compounds have been published.
These are diffraction experiments, thermal and spectroscopic experiments in
addition to those based on heat and electronic transport. These form the raw
materials for this article based on advances this decade. The subject of this
article involves interesting phenomena from the viewpoint of not only physics
but also from the view point 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.
