Transport Properties of Type Ⅱ Sodium-Silicon Clathrates

摘要

We have synthesized the type Ⅱ silicon clathrates Na_1Si_(136) and Na_8Si_(136), and report on the electrical and thermal transport in these materials. The crystal structure consists of a covalently bonded silicon framework in which sodium guest atoms are encapsulated inside the silicon host framework. Differential scanning calorimetry measurements show the compounds decompose above 600℃ to diamond-structure silicon. Temperature dependant electrical resistivity measurements show the specimens to have an insulating character, with magnitudes that decrease with increasing sodium content. For the first time, thermal conductivity measurements on type Ⅱ sodium-silicon clathrates are presented. The thermal conductivity is very low for both specimens, and for Na_8Si_(136) exhibits a clear dip in the range from 50 to 70 K. These data suggest that the "rattling" behavior observed in type Ⅰ clathrates may also be present in type Ⅱ clathrates.