Thermoelectric Properties of the Quaternary Chalcogenides BaCu sub(5.9)STe sub(6) and BaCu sub(5.9)SeTe sub(6)

These quaternary chalcogenides are isostructural, crystallizing in a unique structure type comprising localized Cu clusters and Te sub(2) super(2-) dumbbells. With less than six Cu atoms per formula unit, these materials are p-type narrow-gap semiconductors, according to the balanced formula Ba super(2+)(Cu super(+)) sub(6)Q super(2-)(Te sub(2) super(2-)) sub(3) with Q = S, Se. Encouraged by the outstanding thermoelectric performance of Cu sub(2-x)Se and the low thermal conductivity of cold-pressed BaCu sub(5.7)Se sub(0.6)Te sub(6.4), we determined the thermoelectric properties of hot-pressed pellets of BaCu sub(5.9)STe sub(6) and BaCu sub(5.9)SeTe sub(6). Both materials exhibit a high Seebeck coefficient and a low electrical conductivity, combined with very low thermal conductivity below 1 W m super(-1) K super(-1). Compared to the sulfide-telluride, the selenide-telluride exhibits higher electrical and thermal conductivity and comparable Seebeck coefficient, resulting in superior figure-of-merit values zT, exceeding 0.8 at relatively low temperatures, namely, around 600 K.