β-FeSi 2 and β-Co 0.07 Fe 0.93 Si 2 thermoelectric silicides were synthesized from Fe 2 O 3 and Si powders using a magnesiothermic process. Detailed study of the reaction mechanism by X-ray diraction reveals that liquid Mg is mandatory to initiate the reduction. After completion of the reaction in relatively short time (10 h at 1173 K), the magnesiosynthesized iron disilicides are characterized as powders with grain sizes ranging from 30 to 400 nm and containing a high concentration of stacking faults quantied for 1 Page 1 of 33 ACS Paragon Plus Environment ACS Applied Energy Materials the rst time using a dedicated renement software. The thermoelectric properties of spark plasma sintered pellets with submicrometric grain sizes, high stacking fault density and residual micro-to nanoporosities are presented and compared to corresponding materials synthesized by conventional arc-melting process. Strong thermal conductivity reduction of % at K has been achieved thanks to the mesostructuration induced by the magnesioreduction. It results in an improved maximum gure-of-merit ZT reaching 0.18 at 773 K for β-Co 0.07 Fe 0.93 Si 2 .