Stoichiometric and off-stoichiometric full Heusler Fe2V1−xWxAl thermoelectric systems

The electronic pseudo band gap states and electronic transport of the full-Heusler compound Fe2VAl

“…16,17 Considering the temperature-dependent electrical resistivity r(T) obtained on a variety of Fe 2 VAl samples, a distinct nonmetallic behaviour is derived, with room temperature values around 700 mO cm. 7,12,14,16 The absolute resistivity value coincides reasonably well with the small charge carrier density. Furthermore, the decrease of r(T) with increasing temperature reminds of a semiconductor-like electronic structure.…”

“…In addition, studies on the Hall effect also evidence holes as primary charge carriers with a carrier density of about 4 to 4.8 Â 10 20 cm À3 around room temperature. 2,7,14 These values corroborate the small but finite electronic density of states at the Fermi energy.…”

“…At high temperatures, however, the narrowness of the gap as well as increased electron-phonon scattering leads to a flattening of the r(T) behaviour. 7,12,16 A distinct maximum of the Seebeck coefficient below room temperature seems to confirm this scenario.…”

“…At DE = 0.164 eV the gap is zero and becomes positive for larger shifts; for DE = 0.165 eV it is 0.001 eV. Inspecting eqn (3) and the definition of S(T) according to eqn (7), one realizes that states with energies EðkÞ o m give a positive and states with EðkÞ 4 m a negative contribution to S(T). Summing now over all states by integrating over the entirek-space, the sign of the total S(T) depends on the weight of each electronic state multiplied by the respective band velocities v i (k)v j (k) (which for cubic symmetry is positive definite).…”

“…The excellent mechanical stability of this full-Heusler material, however, goes along with a large value of the thermal conductivity due to both, the high values of the sound velocity (% v s E 5260 m s À1 ) and of the Debye temperature (y D E 650 to 700 K). 7,12 In a series of investigations during the previous two decades, both with respect to density functional theory (DFT), as well as to experimental studies, the following picture emerged: Fe 2 VAl is a non-magnetic and non-metallic material, exhibiting an intriguing band structure with a narrow gap or pseudogap. Depending on the type of DFT calculation, a semimetallic state results in general, as was already shown by one of the very first band structure calculations on Fe 2 VAl by Weinert et al 13 In this calculation, the conduction band minimum around the high symmetry point X of the Brillouin zone falls below the valence band maximum at the G point.…”