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

Abstract

Microstructure and point defects are central to thermoelectric performance in vacancy half-Heuslers.

Advances in half-Heusler alloys for thermoelectric power generation

“…Each of the X V CoSb phases forms with a different vacancy fraction, with the V phase forming preferentially as V 0.87 CoSb, the Nb phase as Nb 0.85 CoSb, while the Ta phase forms close to being 'fully' vacant at Ta 0.81 CoSb. 59 n H is dependent on the vacancy fraction as has been demonstrated for the Nb 0.8+δ CoSb system, 183 so V 0.87 CoSb is highly degenerate while Ta 0.81 CoSb is close to being a semiconductor. 59 There is some level of control that can be achieved, as demonstrated in Nb 0.8+δ CoSb, which can be doped between…”

“…Nevertheless, powder-based routes can give good performing samples, but care has to be taken with precursors, and the lower temperatures also place limitations on grain growth. 59,63,64 Other means of achieving reaction include microwave synthesis and self-propagating combustion synthesis. [65][66][67][68][69] After initial reaction it is common to anneal the sample, typically between 900-1000 °C, to equilibrate the sample and increase homogeneity.…”

“…Perhaps the most significant recent discovery for the n-type HH materials is promising thermoelectric performance in the nominally 19 electron X V CoSb compositions. 59,[183][184][185][186][187][188] As a stoichiometric phase, these materials are expected to be metallic with the possibility of ferromagnetism. 189 In 2016, the material VCoSb was synthesised with S(T) significantly larger than what is typical of a fully metallic sample, although n H was quite large at ~1.6 × 10 22 cm -3 .…”

“…As indicated by defect energy calculations, inversion of X and Y is uncommon (e.g. <1% in X IV NiSn), 57 but there is evidence that this occurs for V 0.8 CoSb and VFeSb, 58,59 where V/Fe and V/Co are (almost) adjacent in the periodic table and hence likely to have similar site preferences.…”

“…HH materials typically have μ H on the order of 10 cm 2 .V -1 .s -1 , with examples of single crystal ZrNiSn as large as 70 cm 2 .V -1 .s -1 and Ta 0.81 CoSb with μ H as low as 1.4 cm 2 .V -1 .s -1 . 59,87 As collection of Hall data has become more frequent, so have detailed investigations into the dominant carrier scattering mechanisms. For semiconductors at low doping concentrations (n H < 10 17 cm -3 ) μ H is expected to be limited by ionised impurity (II) scattering, which has a characteristic μ H ~ T 1.5 behaviour.…”