Ca14AlBi11—a new Zintl phase from earth-abundant elements with a great potential for thermoelectric energy conversion

Abstract

The Zintl phase Ca14AlBi11 has been synthesized and structurally characterized for the first time. Its crystal structure has been carefully determined by single-crystal X-ray diffraction methods, and shown to crystallize in the tetragonal space group I41/acd, No. 142 (Z = 8). Not surprisingly, Ca14AlBi11 adopts the Ca14AlSb11-structure type, although Ca14AlBi11 is the first alumo-bismuthide among the ‘14–1–11’ family whose structure is unequivocally established. In the respective Ca–Al–Bi ternary system, so far, Ca14AlBi11 is only the second identified phase. Electronic structure calculations for this new bismuthide indicate the opening of a small gap at the Fermi level, suggestive of intrinsic semiconducting behavior. Resistivity measurements on as-synthesized single crystalline sample show temperature dependence akin to those of heavily doped semiconductors or the bad metals. In addition to the moderately high electrical conductivity (ρ300 = 1.05 mΩ cm), specimens of Ca14AlBi11 also exhibit excellent thermopower, with high-temperature (HT) values of the Seebeck coefficient approaching and possibly exceeding 200 μV/K. These metrics, coupled with preliminary HT conductivity estimates, are comparable with the best state-of-the-art p-type Zintl thermoelectric materials and are among the best within the known bismuthides with the same atomic arrangements. Therefore, Ca14AlBi11 represents a new platform for the development of a novel, low-cost thermoelectric materials family with first-class transport properties and enhanced thermal stability. Initial data suggest that Ca14AlBi11 is amenable to doping, which opens up many opportunities for tuning charge-carrier concentration and optimizing the transport properties in this new, HT thermoelectric material.