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The solidification kinetics of an alloy from its liquid state forms an underlying basis for microstructural engineering, wherein the state of thermodynamic equilibrium associated with the melt-grown crystal and the quenched amorphous solid denotes the two limits for crystallinity in the alloy synthesis. In this study, we report the implication of the crystalline state on the thermal and electrical transport properties of partially substituted Mn(Si 1−x Al x ) γ by comparing the single crystals melt-grown by the Bridgman method, and polycrystals synthesized from melt spinning (MS) and subsequent spark plasma sintering (SPS). The rapidly solidified alloys exhibited nanocrystalline microstructures in MS ribbons, while melt-grown single crystals displayed characteristics evolution of MnSi striations with limited solubility of Al. It was observed that Al as a p-type dopant enhances the carrier concentration and electrical conductivity, while nanocrystallinity in MS + SPS polycrystals and secondary phases in monocrystals were effective in enhancing the phonon scattering. Maximum zT values of ∼0.54 (±0.05) at 823 K and 0.75 (±0.05) at 873 K were attained for the single crystal (directed perpendicular to the c-axis) and melt-spun polycrystals (along the in-plane direction), respectively. These results present the efficacy of aliovalent Al substitution and demonstrate the critical role of the solidification kinetics in optimizing the carrier concentration and enhancing the phonon scattering in higher manganese silicide crystals for thermoelectric applications.

Higher manganese silicides (HMSs) represented as MnSiγ are generically Nowotny chimney–ladder (NCL) compounds that obey the 14-electron rule due to which their stability and electrical properties are intimately related to the valence electron count (VEC) per number of transition metal atoms. However, owing to the incommensurate composite crystal structure of HMS, most doping/substitution approaches aimed at carrier concentration optimization had remained skewed, leading to limited control over its VEC. In this study, we propose the compensated co-substitution approach for the optimization of thermoelectric properties in NCL phases and demonstrate its efficacy by the co-substitution of the [Mn] subsystem with aliovalent V (p-type) and Ru (n-type) dopants in partially substituted (Mn1–x–y V x Ru y )Siγ single crystals melt grown by the Bridgman method. The modulation vector component (γ) was accurately determined by the Le Bail analysis of the diffraction pattern using a (3 + 1) dimensional superspace approach and is correlated with the electrical transport and VEC of the synthesized samples. A remarkable enhancement in thermoelectric and mechanical performance was attained for the HMS single crystals upon (V, Ru) co-substitution in the direction perpendicular to the c-axis, i.e., along the cleavage plane. The charge compensation and synergistic reduction in lattice thermal conductivity thus result in a peak thermoelectric figure of merit (zT) of ∼0.6 (±0.1) at 823 K, which corresponds to ∼250% enhancement when compared to the pristine HMS single crystal.

Lattice anharmonicity driven by weakening interatomic interactions inherits an intrinsically low lattice thermal conductivity in a crystal. In this work, we demonstrate the efficacy of V (hole-doping) and Fe (electron-doping)...

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