First principles study of some Si-based Type-II Clathrate materials

Since the inorganic clathrate prototypes (Na8Si46, Na24Si136) were discovered in 1965, Si-based Type-II compounds of the form AxRyMzSi136-z (A=Alkali metal; R=Alkali-earth element; M=Ga,Al or Ge; 0≤x+y≤24; 0≤z<136) have been the subject of both many experimental characterization studies and several first-principles computational theory predictions. The primary reason that these materials are of interest to researchers is that their properties make them ideal candidates for use in high performance thermoelectric (TE) materials possessing. Specifically, some of these materials have been shown to posses a large thermoelectric figure of merit (ZT). In this dissertation, I report the results of a systematic first-principles, theoretical and computational study of the electronic, vibrational, and thermal properties of various Type II Si-based clathrates. Specifically, I present the results of such a study of the guest atom containing binary, ternary and quaternary clathrate compounds as well as of some of the alloy mixtures of Si and Ge Type II materials. The materials chosen for this work are the compounds AxSi136 (A=Na,K,Rb,Cs;0<x≤24), (Cs,Rb)8Al8Si128, Cs8Na16Al24Si112 and the alloy Si136-xGex (x=8,32,40,96,104,128).