Unraveling Self‐Doping Effects in Thermoelectric TiNiSn Half‐Heusler Compounds by Combined Theory and High‐Throughput Experiments

Abstract

The control of the carrier concentration is a key topic in the optimization of the thermoelectric power factor. It depends intricately on the defect chemistry of a host phase (here: TiNiSn) and the boundary conditions set by competing phases. The large impact of a slight off‐stoichiometry in the intermetallic half‐Heusler phase TiNiSn makes combinatorial techniques ideally suited for systematic optimization of its thermoelectric performance. In this work, computational thermochemistry, combinatorial synthesis, and high‐throughput characterization are combined to obtain a complete map of the thermoelectric power factor for the Ti–Ni–Sn system. The role of the chemical potential of the constituents in determining the detailed nonstoichiometric composition of the intermetallic half‐Heusler phase TiNiSn is elucidated. This work not only confirms the assumption of a large phase‐width in terms of Ni surplus but also demonstrates that TiNiSn phases with a relatively large Ti surplus can be produced. This can serve as a new route for achieving high carrier concentrations by self‐doping in the ternary system Ti–Ni–Sn. The defect thermochemistry calculations for the carrier concentration are in excellent agreement with the experimental results. The findings of this work suggest new ways of improving the thermoelectric performance of half‐Heusler phases such as TiNiSn.

Manipulation of Ni Interstitials for Realizing Large Power Factor in TiNiSn‐Based Materials

“…Correspondingly, a record‐high power factor (with the peak value ≈100 µW cm −1 K −2 obtained in Ti‐doped NbFeSb‐based materials) highlights the superior electrical performance of the half‐Heusler materials . Among this class of materials, TiNiSn‐based compounds have generated much interest, although their relatively low carrier mobility (below 20 cm 2 V −1 s −1 ) and inferior power factor (≈30 µW cm −1 K −2 ) have impeded their development for high‐performance . Since their counterpart MNiSn (M = Zr, Hf) compounds commonly possess large power factor values (≈50 µW cm −1 K −2 ) resulting from higher carrier mobility (over 30 cm 2 V −1 s −1 ), it is reasonable that the power factor of TiNiSn‐based compounds can be improved via carrier mobility enhancement.…”

Lattice thermal conductivity ofTixZryHf1−x−yNiSnhalf-Heusler alloys calculated from first principles: Key role of nature of phonon modes

“…By including Nivacancy-antisite scattering, obtained from density-functional theory calculations, they predicted κ s in good agreement with experimental numbers for realistic defect concentrations. For instance for ZrNiSn, with a defect-pair concentration of 2.9%, κ is reduced to 7.4 W/m/K, and for 4%, it is reduced to [58][59][60], or the related Ni interstitial defects, which may reduce the κ considerably by scattering phonon modes of higher energy [57]. A high solubility of Ni interstitials has been reported [61][62][63][64][65].…”

Natural off-stoichiometry causes carrier doping in half-Heusler filled tetrahedral structures

“…A number of such compositions were empirically observed in the FTS phase (in the MgAgAs structure) in the past, e.g. ZrNi 1+x Sn 1-y (x=6%, y=0.8%) [43], ZrNi 1+x Sn (x=0-10%) [44], HfNi 1+x Sn (x=0-25%) [44], Ti 1+x Ni 1+y Sn (x=4%, y=3%) [36]. The theoretical question posed here is what causes off-stoichiometry and how they affect carrier doping.…”

“…This issue has only been explored experimentally for just a few FTS compounds including ZrNiSn and TiCoSb [14,20,22,[34][35][36][37][38][39]. The purpose of this paper is to understand the basic physical trends in self-doping (due to natural off stoichiometry) as well as some impurity doping in this family of electronic materials.…”