Effect of synthesis-induced compositional deviations on the Seebeck coefficient of Fe 2 MnAl: Arc melting versus Spark Plasma Sintering

Abstract

The thermoelectric response of Fe 2 MnAl is sensitive to its chemical composition and structural characteristics, which can be influenced by the synthesis route, yet systematic understanding of processing–defect–property relationships remains limited. Here, Fe 2 MnAl, synthesized by arc melting (AM) and spark plasma sintering (SPS), is investigated through combined experimental characterization, density functional theory (DFT), and BoltzTraP2 transport calculations. DFT confirms the half-metallic character of stoichiometric Fe 2 MnAl, with a spin-down band gap of 0.43 eV and a well-defined density of states dominated by transition-metal 3 d orbitals. X-ray diffraction patterns show that both samples predominantly exhibit reflections associated with the L2 1 –type Heusler structure, indicating Fe 2 MnAl as the dominant phase. The analysis therefore focuses on the dominant Fe 2 MnAl-based phase. Compositional analysis revealed that the SPS sample was close to the nominal Fe 2 MnAl stoichiometry (Fe 2.01 Mn 1.00 Al 0.99 ), whereas the AM sample exhibited a Fe-rich and Mn/Al-deficient composition (Fe 2.31 Mn 0.89 Al 0.80 )]. The resulting differences in chemical composition and structural characteristics are accompanied by distinct thermoelectric responses: AM samples exhibit higher positive Seebeck coefficients up to ∼450 K, while SPS samples show earlier sign reversal near ∼360 K with consistently lower magnitudes. To elucidate the microscopic origin of this behavior, BoltzTraP2 calculations were performed on defect-engineered DFT supercells incorporating Fe substitution and Mn/Al vacancies. The results demonstrate that Fe enrichment and Mn/Al deficiency significantly enhance thermopower by reshaping electronic states near the Fermi level, with Fe substitution at Al sites providing the strongest effect. Although our SPS method achieves closer stoichiometric control than AM, the deviations from stoichiometry inherent to AM may facilitate defect-mediated electronic structure engineering, potentially contributing to enhanced thermopower. These findings establish a relationship between synthesis route, stoichiometric deviation, defect-induced electronic structure, and the Seebeck response of Fe 2 MnAl.