Electronic structure and low-temperature thermoelectric transport of TiCoSb single crystals

Abstract

Band structure engineering have a strong beneficial impact on thermoelectric performance, where theoretical methods dominate the investigation of electronic structures. Here, we use angle-resolved photoemission spectroscopy (ARPES) to analyze the...

Realization of Band Convergence in p-Type TiCoSb Half-Heusler Alloys Significantly Enhances the Thermoelectric Performance

“…The Ioffe–Pisarenko plot (Figure b) shows that m DOS * increases with increasing the Sn content up to x = 0.03, with each sample following a distinct curve. The highest value of ∼ 4 m e agrees with that determined from the Seebeck coefficient in single-crystalline TiCoSb, which has been taken as an indication of the convergence of the valence bands at the Γ and L points . Further increasing the Sn content to x = 0.04 lowers α from 164 to ∼142 μV K –1 at RT.…”

“…The power factor (PF), shown in Figure a, increases monotonically up to x = 0.03 for which an optimum value of ∼1.94 mW m –1 K –2 is reached at 825 K. This peak value is comparable to that obtained in metallic, single-crystalline TiCoSb and in optimized HHs at high temperatures. , …”

“…At room temperature, the Seebeck coefficient (α) increases monotonically with x up to 0.03, an upper limit above which a reverse trend is observed (Figure a). To better understand this behavior, the theoretical Ioffe–Pisarenko curve has been calculated by a single-parabolic band (SPB) model with acoustic phonon scattering that was shown to dominate the transport in single crystals according to the following relation , S = k normalB e ( 2 F 1 false( η false) F 0 false( η false) − η ) where the Fermi integral of order j is defined as F j ( η ) = prefix∫ 0 ∞ scriptϵ j d scriptϵ 1 + exp ( ϵ − η ) . The chemical carrier concentration is expressed as n = 4 π true( 2 m DOS * k normalB T h 2 true) 3 / 2 F 1 / 2 ( η ) , while the Hall carrier concentration is given by n normalH =…”

“…At room temperature, the Seebeck coefficient (α) increases monotonically with x up to 0.03, an upper limit above which a reverse trend is observed (Figure a). To better understand this behavior, the theoretical Ioffe–Pisarenko curve has been calculated by a single-parabolic band (SPB) model with acoustic phonon scattering that was shown to dominate the transport in single crystals according to the following relation , where the Fermi integral of order j is defined as . The chemical carrier concentration is expressed as , while the Hall carrier concentration is given by , with the Hall factor .…”

“…While m b * can be increased by distortion of the DOS through band flattening or resonant levels, N v mainly depends on the crystal structure symmetry and can be increased by convergence of several band extrema offset by an energy difference of few k B T . , These multiple degenerate valleys improve the m DOS * without adversely impacting the mobility of the charge carriers, provided that intervalley scattering is not strong . Band convergence may easily occur in materials with a highly symmetric crystal structure and Fermi surface forming isolated pockets at low symmetry points. , This effect has been successfully induced in many systems such as CoSb 3 , Mg 2 Si, SnTe, ,, InSe, Bi 2 Te 3 , , PbTe, , and half-Heuslers. − In the p-type HH compound TiCoSb, angle-resolved photoemission spectroscopy (ARPES) measurements on single crystals have evidenced the presence of several valence bands at the L and Γ points of the Brillouin zone with a very small energy offset of ∼0.05 eV . These results suggest that convergence of these bands may be achieved upon appropriate substitutions, thereby providing a straightforward mean to enhance the TE performance of this compound.…”