Observation of Anomalously High Seebeck Coefficients in the Family of Zintl Phase Semiconductors Ca₁₀RECdSb₉ (RE = Rare-Earth Metal)

Abstract

This work reports the synthesis, structure, and thermoelectric transport properties of the hole-dominated semiconductors with a general formula Ca10 RECdSb9 (RE = Y, Ce, Nd, Sm, and Lu). These materials feature unexpectedly high Seebeck coefficients, up to 650 μV/K at 600 K, and electrical resistivities on the order of 1–1000 mΩ·cm, which is an indication of a degenerate semiconducting state. For example, the Seebeck coefficient observed in Ca10NdCdSb9 (309 μV/K at 600 K) is accompanied by low and metallic-like electrical resistivity (6 mΩ·cm) and a carrier concentration of n = 4.68 × 1020 cm–3. The calculated power factor PF in Ca10NdCdSb9 is 0.88 μW/cm·K2 at 300 K, and calculations based on the single parabolic band model indicate that an optimum PF opt of 1.68 μW/cm·K2 can be achieved at that temperature for a carrier concentration of n opt = 6.62 × 1019 cm–3. The estimated thermoelectric figure of merit zT in this material when properly tuned is expected to surpass zT = 1 at 600 K.

Exploiting the fraternal twin nature of thermoelectrics and topological insulators in Zintl phases as a tool for engineering new efficient thermoelectric generators

“…[36][37][38] This interest led to the rapid expansion of Zintl phases into the realm of transition and rare-earth elements; when such elements of open core-shells are incorporated, the attendant structural diversity and physical properties can reach new heights. 20,30,[39][40][41][42][43][44] The actualization of closed-shell electronic configuration in these materials brings about their characteristic features as semiconductors or insulators as well as the diamagnetic or temperature independent paramagnetic behavior. 26 The Zintl phases combine the features of narrow band gap, intricate atomic bonding and amenability to a high degree of disorder and present a fertile playground to explore the various electronic and transport properties of materials of interests ranging from the topological phases of matter 45,46 to applications in thermoelectrics 47 and photovoltaics.…”

“…36–38 This interest led to the rapid expansion of Zintl phases into the realm of transition and rare-earth elements; when such elements of open core-shells are incorporated, the attendant structural diversity and physical properties can reach new heights. 20,30,39–44…”

Crystal and electronic structure of the ternary Zintl bismuthide BaLiBi

“…Numerous studies have been devoted to crystal-chemical design of new Zintl compounds with enhanced thermoelectric figure of merit. [2][3][4][5][6][7] Thanks to their narrow bandgaps, these materials exhibit high electrical conductivity, while their complex crystal structures and/or presence of heavy elements in the composition yield low thermal conductivity, thereby providing a favorable combination of transport properties. High crystal symmetries and presence of low-dimensional crystal structural units, such as polyanionic chains or layers, often result in high band degeneracies and formation of bands with low dispersion, which can ultimately lead to steep electronic density of states and high Seebeck coefficients, beneficial for thermoelectric performance.…”

“…Since the discovery of high thermoelectric performance in Yb 14 MnSb 11 , [1] Zintl phases have established themselves as promising materials for thermoelectric applications. Numerous studies have been devoted to crystal‐chemical design of new Zintl compounds with enhanced thermoelectric figure of merit [2–7] . Thanks to their narrow bandgaps, these materials exhibit high electrical conductivity, while their complex crystal structures and/or presence of heavy elements in the composition yield low thermal conductivity, thereby providing a favorable combination of transport properties.…”