Synthesis of Zintl phase metal silicide thermoelectric materials via MG/Zn flux

We carried out reactions of silicon with barium and strontium in excess Mg/Zn flux to synthesize (Ba/Sr)_5+xMg_19-xSi₁₂ to investigate the effect of varying Ba/Sr ratio on site mixing, structure transitions, and thermoelectric properties. (Ba/Sr)_5+xMg_19-xSi₁₂ compounds with 0 < x < 3 are charge-balanced Zintl phases which adopt the hexagonal Ho₅Ni₁₉P₁₂structure type. Single-crystal X-ray diffraction data for Ba_5.36Mg_18.64Si₁₂, Ba_4.60Sr_2.88Mg_16.51Si₁₂, Ba_4.10Sr_3.75Mg_16.16Si₁₂, Ba_3.9Sr_4.23Mg_15.87Si₁₂, Ba_2.81Sr_5.19Mg₁₆Si₁₂, Ba_2.09Sr_5.71Mg_16.20Si₁₂, and Sr_7.85Mg_16.15Si₁₂show occupation of barium and strontium cations in Ho sites, while strontium mixes with magnesium on a specific Ni site. Elemental analysis supports the site mixing and also confirms that zinc from the flux is not incorporated. Refinement of powder XRD data of products indicated they are single phase throughout the sample. Thermoelectric measurements were carried out to test the relationship between mixed crystallographic sites and electrical and thermal transport properties. To induce more disorder in the structure, incorporation of other divalent cations including Ca, Eu, and Yb is being explored.