Fe2MnSixGe1−x: influence thermoelectric properties of varying the germanium content

Abstract

The semi-classical Boltzmann theory, as implemented in the BoltzTraP code, was used to study the influence of varying the germanium content on the thermoelectric properties of the Heusler compounds, Fe₂MnSi and Fe₂MnGe. The electrical conductivity (σ/τ), the Seebeck coefficient (S), the electronic power factor (S²σ), the electronic thermal conductivity (κ_e), the electronic heat capacity c_el(T_el), and the Hall coefficient (R_H), as a function of temperature at certain values of chemical potential (μ) with constant relaxation time (τ), were evaluated on the basis of the calculated band structure using the standard Boltzmann kinetic transport theory and the rigid band approach. The increase/reduction in the electrical conductivity (σ = neμ) of Fe₂MnSi_xGe_1−x alloys is attributed to the density of charge carriers (n) and their mobility (μ = eτ/m_e). The S for Fe₂MnGe is negative over the entire temperature range, which represents the n-type concentration. Whereas Fe₂MnSi shows a positive S up to 250 K and then drops to negative values, which confirms the existence of the p-type concentration between 100–250 K. Fe₂MnSi_0.25Ge_0.75/Fe₂MnSi_0.5Ge_0.5/Fe₂MnSi_0.75Ge_0.25 possess positive S up to 270/230/320 K and then drop to negative values. The power factor of Fe₂MnGe rapidly increases with increasing temperature, while for Fe₂MnSi it is zero up to 300 K, and then rapidly increases with increasing temperature. The S²σ of Fe₂MnSi_0.25Ge_0.75 is zero between 250–350 K, whereas Fe₂MnSi_0.5Ge_0.5 possesses a zero S²σ of up to 320 K. Fe₂MnSi_0.75Ge_0.25 has a zero S²σ between 200 and 500 K. The electronic thermal conductivity (κ_e) and the electronic heat capacity c_el(T_el) increases with increasing temperature. The parent compounds (Fe₂MnGe and Fe₂MnSi) show the highest positive value of the Hall coefficient R_H at 100 K, and then drop to negative values at 260 K. On the other hand, the R_H for Fe₂MnSi_0.25Ge_0.75, Fe₂MnSi_0.5Ge_0.5 and Fe₂MnSi_0.75Ge_0.25 alloys exhibit negative R_H along the temperature scale. The behavior of R_H is attributed to the concentration of the charge carriers and their mobility.