Band structure engineering in Fe–Sb based lanthanide filled p-type skutterudites RFe4Sb12 (R = Nd, Sm) to enhance the Seebeck coefficient and thermoelectric figure of merit

Abstract

Structural, thermodynamic, electronic, and thermoelectric properties of two pure ternary skutterudites, NdFe4Sb12 and SmFe4Sb12, and their doped counterparts, Sm-doped NdFe4Sb12 and Nd-doped SmFe4Sb12, have been investigated using full potential linearized augmented plane wave formalism under density functional theory. In doped systems, the central lanthanide atom was replaced by a different filler atom. Thermodynamic parameters indicate that all the materials are stable, sufficiently hard, and will have a high melting point. Band profiles reveal their semimetallic nature with a pseudo-bandgap above the Fermi level and crossing of the Fermi level of one or more bands. The facts that the trivalent fillers do not provide enough electrons required for charge compensation of Fe4Sb12 and the Fermi levels are well inside the valence band also predict their p-type nature. The splitting of DOS of the f-electrons of the filler atoms into both spin channels implies their ferromagnetic nature. The Sm-doped system exhibits the highest magnetic moment because of the much lower anti-ferromagnetic moment of Fe. Between the pure compounds, the lighter filler atom-based NdFe4Sb12 exhibits a higher ZT value because of the higher population density of states and higher concentration of degenerate flatbands. Contrary to recent predictions, both the doped systems show higher ZT than the pure ones. However, the presence of larger pseudo-bandgaps in both spin channels and two peaks just above the Fermi level in the majority spin channel in the lighter Nd-doped system results in the enhanced Seebeck coefficient, reduced thermal conductivity, and the maximum ZT value of 0.90 at 1000 K.