Origin of large thermoelectric power in off-stoichiometric Fe₂VAl-based alloys

Abstract

Abstract. The valence-band and core-level soft x-ray photoelectron spectroscopy has been applied to investigate the origin of the large thermoelectric power in the off-stoichiometric and partially-substituted Fe-based Heusler-type alloys Fe 2-x-y Ir y V 1+x Al and Fe 2-x V 1+x-y Ti y Al. The non-rigid-band-like change in the valence-band electronic structure near the Fermi level E F is found for the off-stoichiometric change x in Fe and V concentrations, whereas the rigid-bandlike shift of E F is recognized on the Ir or Ti substitution y. The non-rigid-band-like change may be induced by the excess-V or Fe (anti-site defects)-derived states in the pseudogap of Fe 2 VAl, while the rigid-band-like shift may be caused by a common band formation, where the d states of the substituted elements are incorporated into the main Fe-V 3d bands. The V2p core-level spectra consistently show the satellite structure due to the excess V as the anti-site defect. A model electronic structure is presented for explaining the observed enhancement of their thermoelectric power.

Angle-Resolved Photoemission Analysis of Electronic Structures for Thermoelectric Properties of Off-Stoichiometric Fe_2–xV_1+xAl Alloys

“…2. It has been also found that the simultaneous substitution by a forth element in off-stoichiometric specimens leads to increased thermoelectric power 17,18,[22][23][24] , which shows rigid-band-like dependence on the substitution from the non-rigid-band-like value of the off-stoichiometric alloy as a starting point (broken curve in Fig. 2) 18) .…”

“…We have experimentally investigated the total electronic density of states of off-stoichiometric Fe 2-x V 1+x Al alloys with angle-integrated soft x-ray photoelectron spectroscopy to clarify their electronic structures for thermoelectric applications 17,23,24) . Although it is dif cult to study the unoccupied states by photoelectron spectroscopy, the spectral shape near E F of the valence band of Fe 2-x V 1+x Al changes from a p-type semiconductor-like steep increase to a small shoulder feature representing the pseudogap as V increases, which qualitatively explains the observed change from p-to n-type thermoelectric power (see Fig.…”

“…5). On the basis of these experimental and theoretical results, we proposed a model electronic band structure explaining the x-dependence of the thermoelectric power 24) . Herein, we discuss the experimental results of angle-resolved photoelectron spectroscopy for the off-stoichiometric alloy Fe 2.05 V 0.95 Al to clarify its valence-band electronic structure, particularly the band structure near E F and the states induced by the antisite defects, and discuss the effect of the Fe/V off-stoichiometry on the electronic structure and thermoelectric properties.…”

Unexpected p-type thermoelectric transport arising from magnetic Mn substitution in Fe2V1−xMnxAl Heusler compounds

“…The electronic structure near the Fermi level is found to be drastically altered by off-stoichiometry, while doping with Ti has a more moderate effect in a rigidband-like manner [79]. In the case of off-stoichiometric Fe 2-x V 1+x Al-based Heusler alloys, Soda et al proposed that the excess V or Fe (antisite) defects may induce impurity states near the valence band edge, resulting in a potential enhancement of the Seebeck coefficient [80]. To elucidate the anomalous p-type Seebeck coefficient in nominally n-doped Fe 2 V 1-x Mn x Al, we have performed extensive density functional theory calculations of the alloy-averaged densities of states of substituted Fe 2 V 1-x Mn x Al, which are summarized in the subsequent section.…”

Thermoelectric properties of composition-controlled Fe2TiSi-based full-Heusler thin films

“…The decrease of Fe from the stoichiometric value of 2.00 is known to create antisites and consequently shift the dependence of S on the VEC to the lower VEC side by creating a steep DOS slope in the conduction band near E F due to the emergence of the impurity sites. 23,24) Figure 2(c) shows the dependence of S and ρ of Fe 2 TiSi+V(2)+X(n) on the VEC when the total number of chips (n+2) was fixed to five for making the Fe amount a negligible change and the added chips (X) were V, Ti, Al, and Si. The Fe amount ranged from 1.98 to 1.99, which is almost the same as the stoichiometry.…”