"In addition to their ground state, diamond structure, Si, Ge and Sn can form crystalline solids called clathrates. As in the diamond structure, in the clathrates, the Group IV atoms are tetrahedrally coordinated and sp3 covalently bonded to their neighbors. However, the clathrates contain pentagonal atomic rings and have open-framework lattices containing 20-, 24- and 28-atom “cages”. ... The pure clathrates are semiconductors.
The cages can contain weakly bound impurities (“guests”), usually alkali or alkaline earth atoms. The choice of guest may be used to tune the material properties. The guests are electronic donors, but because of their weak bonding, they have small effects on the electronic band structures. However, they can produce low frequency vibrational (“rattling”) modes which can strongly affect the vibrational properties. Some guest-containing clathrates have been shown to be excellent candidates for thermoelectric applications precisely because the guests only perturb the electronic properties weakly, while strongly affecting the vibrational (heat transport) properties. For thermoelectric applications, semiconductor materials are needed. When all cages are filled, the clathrates become semi-metallic due to the excess valence electrons of the guests. To compensate for this, Group III atoms (usually Ga or In) are substituted on the framework sites."
More at Physica B: Condensed Matter, Volumes 401-402, 15 December 2007, Pages 695-698, Proceedings of the 24th International Conference on Defects in Semiconductors, Rattling “guest” impurities in Si and Ge clathrate semiconductors, C.W. Myles, K. Biswas and E. Nenghabi
The Latin word clatrum means lattices; bars; grate; railings -> Clatratus