Unusual Phonon Scattering and Nanostructuring Implications on Thermoelectric Properties of the Incommensurate MnSi _γ Nowotny Chimney Ladder Phase

Contemporary material design in thermoelectric research entails microstructure control and interface manipulation to attain thermal conductivity reduction, for which nanostructuring approaches are extensively assumed. Herein, we present the implications of grain size reduction and sintering on structural parameters, microstructural features, and transport properties of nanostructured silicon-rich manganese silicide (MnSix) bulk polycrystals synthesized employing arc-melting, ball milling, and spark plasma sintering. The incommensurate periodicities and phase evolution in nanostructured MnSi1.74, a prominent Nowotny chimney ladder (NCL), were analyzed employing a (3 + 1) dimensional superspace approach and were corroborated by electron micrographs. It was inferred that nanostructuring employing ball milling inevitably led to a disintegrated microstructure with spatial fluctuations in the Mn/Si ratio, comprising Mn-rich (Si-deficient) and Mn-deficient (Si-rich) regions in varying proportions. The acoustic-optical phonon coupling, unusual phonon scattering, and textural anisotropy were established for the Mn–Si phase under Si-rich conditions, to determine the prospects of nanostructuring in MnSiγ for thermoelectric applications.