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

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 (sigma/tau), the Seebeck coefficient (S), the electronic power factor (S-2 sigma), the electronic thermal conductivity (k(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 (mu) with constant relaxation time (tau), 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 (sigma = ne mu) of Fe2MnSixGe1-x alloys is attributed to the density of charge carriers (n) and their mobility (mu = e tau/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. Fe2MnSi0.25Ge0.75/Fe2MnSi0.5Ge0.5/Fe2MnSi0.75Ge0.25 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 S-2 sigma of Fe2MnSi0.25Ge0.75 is zero between 250-350 K, whereas Fe2MnSi0.5Ge0.5 possesses a zero S-2 sigma of up to 320 K. Fe2MnSi0.75Ge0.25 has a zero S-2 sigma between 200 and 500 K. The electronic thermal conductivity (k(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 Fe2MnSi0.25Ge0.75, Fe2MnSi0.5Ge0.5 and Fe2MnSi0.75Ge0.25 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.