Special features of conductivity mechanisms in heavily doped n-ZrNiSn intermetallic semiconductors

Introduction of resonant states and enhancement of thermoelectric properties in half-Heusler alloys

“…For example, self-consistent tight-binding linear muffin-tin orbital (TB-LMTO) calculations have predicted that a peak appears within the band gap of semiconducting VFeSb, TiCoSb, and TiNiSn compounds when as few as 3.125% and as many as 25% of the atoms on the lower valent transition metal site are replaced with manganese, while substitution of transition metals to the other two sites was found to be energetically unfavorable [7]. Similar results were found with Korringa-Kohn-Rostoker calculations performed with the coherent potential and local density approximations (KKR-CPA-LDA) on ZrNiSn with up to 10% cobalt doped to the nickel site, although no subsequent increase in ZT was reported [8]. More recently, a systematic study of gap size and thermoelectric properties has suggested a model for the location of dopant-induced peaks within the band gap region in the case of 3d transition metal substitution to either transition metal site of Hf 0.75 Zr 0.25 NiSn [9].…”

“…As has been proposed before [7,8,9], these results suggest that the doping of transition metals into half-Heusler systems introduces a local feature in the density of states near to or within the band gap region. This addition of impurity-induced states near the Fermi energy contributes toward a greater Seebeck coefficient at room temperature, while at higher temperatures n-type carriers are promoted into the conduction band, away from the local enhancement and therefore minimizing its benefit.…”

“…While the Seebeck coefficient of this compound is improved by a factor of nearly 20% by 450 K as a result of doping, no effects of this enhancement are present in measurements taken at temperatures of 600 K or higher, suggesting that while the density of states is significantly altered at or near the Fermi energy, this modification is only relevant across a narrow energy scale -on the order of 50 meV, which is small compared to both the calculated 0.5 eV and measured 0.22 eV band gaps [9]. Hence it appears that the resonance in lightly vanadium doped Hf 0.75 Zr 0.25 NiSn is qualitatively similar to Mahan and Sofo's Dirac function resonance and thus is remarkably distinct from the gap-spanning dopant-induced peaks predicted to form in half Heusler compounds by ab-initio calculations that considered significantly larger concentrations of manganese and cobalt dopant atoms [7,8]. In these cases, the resonance broadens with greater dopant concentration to the rapid detriment of thermoelectric properties and the consequent increase of disorder and of carrier scattering [9].…”

“…The results of ab-initio calculations suggest that even in undoped ZrNiSn, there exists a low but non-zero density of states within the gap due to anti-site disorder, which pulls the Fermi energy up from the top of the valence band toward the middle of the gap. Furthermore, dopant-induced pinning or saturation of the Fermi energy akin to the above observations has been reported previously in transition metal-doped half-Heusler compounds [7,8,9]. This phenomenon can be explained as a direct effect of the hybridization of an unstable vanadium d resonance into a stable electronic state capable of holding some number of electrons -depending on impurity concentration and degree of d-orbital hybridization -and thereby preventing the Fermi energy from reaching the majority conduction band until these resonant levels are overfilled by electrons contributed by another dopant, such as antimony.…”

Resolving the true band gap of ZrNiSn half-Heusler thermoelectric materials

“…Based on ab initio calculations, a high mobility value for n-type ZrNiSn is expected owing to their light conduction band (CB). The valence band (VB), however, has been shown to have a higher effective mass, and so by substituting Zr with Sc, one might expect a lower mobilities (in magnitude) for p-type materials as the Fermi energy gets shied into the VB at G. 25,37 This is consistent with our experimental results, shown in Fig. 2b; the Hall mobilities of the heavily Scdoped samples are much lower in magnitude than the n-type ZrNiSn undoped sample.…”