Charge-Compensated (V, Ru) Co-Substitution in Higher Manganese Silicide Single Crystals for Enhanced Thermoelectric and Mechanical Performance

Higher manganese silicides (HMSs) represented as MnSi gamma are generically Nowotny chimney-ladder (NCL) com-pounds 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-yVxRuy)Si gamma single crystals melt grown by the Bridgman method. The modulation vector component (gamma) 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 similar to 0.6 (+/- 0.1) at 823 K, which corresponds to similar to 250% enhancement when compared to the pristine HMS single crystal.