Synthesis of Zintl Phase Metal Silicide Thermoelectric Materials in Magnesium/Zinc Flux

Zintl phases have potential applications as thermoelectric materials for power generation and cooling owing to their complex crystal structures and unique electronic properties. We carried out reactions of silicon with barium and strontium in excess Mg/Zn flux to synthesize (Ba/Sr)_5+xMg_19-xSi₁₂ Zintl phases, investigating the effect of varying Ba/Sr ratio on site mixing and thermoelectric properties. (Ba/Sr)_5+xMg_19-xSi₁₂ compounds with 0 < x < 3 are charge-balanced Zintl phases which adopt the hexagonal Ho₅Ni₁₉P₁₂ structure type (space group P6̅2m). Density of states calculations indicate that these materials are semimetals. Single-crystal X-ray diffraction data and elemental analysis for Ba₅Mg₁₉Si₁₂, Ba_4.86Sr_2.94Mg_16.20Si₁₂, Ba_3.63Sr_4.20Mg_16.17Si₁₂, Ba_1.93Sr_5.99Mg_16.08Si₁₂, and Sr_7.82Mg_16.18Si₁₂ show occupation of barium and strontium cations in Ho sites, while strontium mixes with magnesium on a specific Ni site. Powder XRD data of products show that they are single phase throughout the sample. Thermoelectric measurements indicate that increasing strontium content and mixing on three cation sites decreases thermal conductivity; it is hypothesized that improved overall thermoelectric behavior is likely due to the rattling of the Sr cations in their positions.