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

Abstract

p-Type Fe₂VAl-based thermoelectrics have been much less investigated compared to their respective n-type counterparts. Thus, it is crucial to identify novel doping strategies to realize enhanced p-type Fe₂VAl Heusler compounds. In the current study, the effect of Mn substitution in Fe₂V_1−xMn_xAl is investigated with respect to temperature-dependent electronic transport as well as temperature- and field-dependent magnetic properties. We find an anomalous and unexpected p-type Seebeck coefficient for nominally n-doped Fe₂V_1−xMn_xAl over an extremely large range of concentrations up to x = 0.6. Using density functional theory (DFT) calculations, this is traced back to distinct modifications of the electronic structure, i.e., localized magnetic defect states (m = 2.43μ_B) at the valence and conduction band edges, and a concomitant pinning of the Fermi level within the pseudogap. Furthermore, we were able to further optimize the thermoelectric properties by co-doping Al antisites in off-stoichiometric Fe₂V_0.9Mn_0.1Al_1+y, yielding sizeable values of the power factor, PF = 2.2 mW K⁻² m⁻¹ in Fe₂V_0.9Mn_0.1Al_1.1 at 350 K, and figure of merit, ZT ∼ 0.1 for highly off-stoichiometric Fe₂V_0.9Mn_0.1Al_1.5 at T = 500 K. Our work underlines the prospect of engineering Fe₂VAl-based Heusler compounds via magnetic doping to realize enhanced p-type thermoelectris and encourages studies involving other types of co-substitution for Mn-substituted Fe₂V_1−xMn_xAl.