Enhanced Thermoelectric Performance in 18‐Electron Nb_0.8CoSb Half‐Heusler Compound with Intrinsic Nb Vacancies

Abstract

Typical 18-electron half-Heusler compounds, ZrNiSn and NbFeSb, have been identified as promising high temperature thermoelectric materials. NbCoSb with nominal 19 valence electrons, which is supposed to be metallic, has recently been reported to also exhibit thermoelectric properties of a heavily doped n-type semiconductor. Here we experimentally demonstrate, for the first time, that This article is protected by copyright. All rights reserved. 2 the nominal 19-electron NbCoSb is actually the composite of 18-electron Nb 0.8+ CoSb (0 ≤ < 0.05) and impurity phases. Single phase Nb 0.8+ CoSb with intrinsic Nb vacancies, following the 18-electron rule, possesses improved thermoelectric performance, and the slight change in the content of Nb vacancies has a profound effect on the thermoelectric properties. The carrier concentration can be controlled by varying the Nb deficiency, and the optimization of the thermoelectric properties can be realized within the narrow pure phase region. Benefiting from the elimination of impurity phases and the optimization of carrier concentration, thermoelectric performance is remarkably enhanced by ~100% and a maximum zT of 0.9 is achieved in Nb 0.83 CoSb at 1123 K. This work expands the family of half-Heusler thermoelectric materials and opens a new avenue for searching for nominal 19electron half-Heusler compounds with intrinsic vacancies as promising thermoelectric materials.

Low thermal conductivity and promising thermoelectric performance in AxCoSb (A = V, Nb or Ta) half-Heuslers with inherent vacancies

“…The carrier mobility for the Nb sample is largest at m H = 4.2 cm 2 V À1 s À1 , which is comparable to the highest literature values. 11,27 The mobility is lower for the V sample with m H = 1.2 cm 2 V À1 s À1 , which is partially due to its higher m* and n H but is also likely to be linked to the observed porosity. The magnitude is in keeping with reported data for nominally stoichiometric VCoSb.…”

“…The value for the Nb sample is in good agreement with m* = 8-10m e from the literature. 11,27 The decrease of m* = N v 2/3 m b signals either a decrease in valley degeneracy (N v ) or increasingly dispersive electronic bands (reduced m b ) for the pockets that contribute the electrical transport. The more diffuse 4d and 5d orbitals for the heavier A elements are known to afford better orbital overlap and increased bandwidths, which is in keeping with the observed trend.…”

“…6 This was resolved through studies of NbCoSb that demonstrated the formation of Nb vacancies, leading to Nb x CoSb (x B 0.85) compositions with VEC B 18.25. [9][10][11] Varying the nominal Nb content between x = 0.8 (VEC = 18) and x = 1 (VEC = 19) affords a systematic change in carrier concentration (n H ) and a means of optimising the thermoelectric power factor, S 2 /r, where S is the Seebeck coefficient and r is the electrical resistivity. 11 The optimised power factor of S 2 /r = 2.4 mW m À1 K À2 and low thermal conductivity (k) resulted in a highly promising thermoelectric figure of merit, ZT = (S 2 /rk)T = 0.9 at 1123 K, demonstrating that vacancies are a new, useful route to tune the individual thermoelectric parameters and achieve good performance.…”

Complex Band Structures and Lattice Dynamics of Bi2Te3‐Based Compounds and Solid Solutions

“…After decades of stagnation, the research on TE field revived due to the development of the new concepts, such as the quantum confinement and the phonon‐glass‐electron‐crystal (PGEC), both proposed in 1990s. In recent years, several semiconductors, such as half‐Heusler compounds, filled skutterudites, clathrates, Cu 2 (Te, Se, S), Mg 2 (Si, Sn, Ge), chalcopyrite, BiCuSeO, MgAgSb, SnTe, GeTe, and Mg 3 Sb 2 have been identified as promising new high‐performance TE materials. These materials possess high zT values exceeding the barrier of unity easily and sometimes even above 2.…”

Substitution Versus Full-Heusler Segregation in TiCoSb

“…In general, HHs are characterised by large power factors (S 2 /ρ), which enables large amounts of power to be extracted from the generators, but are limited by relatively large κ values, which reduces the overall device efficiency [1,2]. The current best materials are based on XNiSn (n-type) [4][5][6][7], XCoSb (p-type) [8,9] (X = Ti, Zr, Hf), and X FeSb (p-type) [10][11][12] (X = V, Nb, Ta), while Nb 0.8-0.9 CoSb alloys have recently emerged as promising new n-type compositions [13,14].…”