Synthesis and thermodynamics properties of alkaline metals filled CoSb3 skutterudites

Alkaline metal (AM) atoms filled CoSb₃ skutterudites are synthesized by melting method and sintered by spark plasma sintering. The maximal filling fraction or filling fraction limit (FFL) of AM (AM=Li, Na, K, Cs) in CoSb₃ are estimated experimentally and show good agreement with our previous calculations based on density functional method. The effect of thermodynamics properties on FFL, crystallographic properties and microstructure are investigated. Among all the AM-filled CoSb₃ systems, Na shows the highest experimental FFL of ~65% due to the relatively stable chemical properties of Na and NaSb_x binary compounds. Furthermore, Na_yCo₄Sb₁₂ shows the best crystallization and purity under the same synthesis approach. In the cases of the end members Li and Cs, both our theoretical and experimental results show that the filling levels in CoSb₃ are infinitesimal due to the extremely low formation enthalpies of LiSb_x and CsSb_x compounds.