Thermoelectric Properties of Ba1.9Ca2.4Mg9.7Si7: A New Silicide Zintl Phase with the Zr2Fe12P7 Structure Type

Ba1.9Ca2.4Mg9.7Si7 was grown as large crystals from the reaction of silicon with barium and calcium in Mg/Al flux. This compound is a charge-balanced Zintl phase with the Zr2Fe12P7 structure type in hexagonal space group P6̅ (a = 11.196(2) Å, c = 4.4595(9) Å); barium occupies the Zr sites, the lighter alkaline earths (Mg, Ca) mix on the Fe sites, and silicon occupies the phosphorus sites. An isostructural germanide (Ba1.2Sr0.9Mg11.9Ge7, a = 11.064(1) Å, c = 4.3709(6) Å)) was similarly synthesized from reactions of germanium with barium and strontium in Mg/Al flux. Density of states calculations indicate these phases are semimetals, in agreement with their charge-balanced nature. Measurements of electrical resistivity, thermal conductivity, and Seebeck coefficient were carried out from 300–1000 K to investigate the thermoelectric properties of Ba1.9Ca2.4Mg9.7Si7. This compound is an n-type semimetal with low thermal conductivity (2 W/(m·K)), moderate Seebeck coefficient (−200 μV/K at 900 K), and a thermoelectric figure of merit ZT of 0.35 at 900 K.