Anisotropic correlations in higher manganese silicides

Anisotropic correlations are vital for understanding the thermoelectric transport properties of higher manganese silicides (HMSs). In this work, we establish the implications of anisotropy on thermoelectric performance by examining the orientational variants in melt-grown single crystal and directional configurations of spark plasma sintered polycrystalline specimens of HMS. Single crystal specimen aligned perpendicular to the c-axis (i.e., along the cleavage ab-plane) exhibited high electrical and thermal conductivity, while the maximum Seebeck coefficient was measured along the c-axis direction. Similarly, polycrystalline specimens exhibited a weakened anisotropy with an enhanced weighted mobility and reduced thermal conductivity than their monocrystal counterpart resulting in a relatively higher thermoelectric figure-of-merit (zT). The characteristic MnSi striations were found to be predominantly grown perpendicular to the c-axis in melt-grown single crystals while peritectic decomposition during sintering is commonly observed in sintered polycrystals thereby emanating an inherently disordered and modulated lattice structure in the synthesized HMS crystals. It was observed that the zT for single and polycrystalline HMSs could be erroneously estimated if charge and phonon transport properties are measured in different directions. This study offers newer insights into the structure-property relationships of HMSs and demonstrates the need for more careful considerations in addressing the anisotropic behavior and microstructural evolutions that prevails in HMS crystals.