Structure and physical properties of the thermoelectric skutterudites ${\mathrm{Eu}}_y{\mathrm{Fe}}_{4-x}{\mathrm{Co}}_x{\mathrm{Sb}}_{12}$

Compounds from the solid solution ${\mathrm{Eu}}_y{\mathrm{Fe}}_{4-x}{\mathrm{Co}}_x{\mathrm{Sb}}_{12}$ were synthesized and found to crystallize in the partially filled skutterudite-type structure ${\mathrm{LaFe}}_4{\mathrm{P}}_{12}.$ The maximal Eu content gradually decreases from practically full occupancy $y=0.83\mathrm{}$ in ${\mathrm{Eu}}_{0.83}{\mathrm{Fe}}_4{\mathrm{Sb}}_{12}$ to $y=0.44\mathrm{}$ for ${\mathrm{Eu}}_{0.44}{\mathrm{Co}}_4{\mathrm{Sb}}_{12}.$ As a function of Fe/Co substitution in ${\mathrm{Eu}}_y{\mathrm{Fe}}_{4-x}{\mathrm{Co}}_x{\mathrm{Sb}}_{12}$ the magnetic transition temperature decreases from 84 K for ${\mathrm{Eu}}_{0.83}{\mathrm{Fe}}_4{\mathrm{Sb}}_{12}$ to 8 K for ${\mathrm{Eu}}_{0.44}{\mathrm{Co}}_4{\mathrm{Sb}}_{12}.$ At lower Eu content, ${\mathrm{Eu}}_{0.2}{\mathrm{Co}}_4{\mathrm{Sb}}_{12},$ long-range magnetic order disappears. Concomitantly, the Eu valence changes from almost divalent Eu in ${\mathrm{Eu}}_{0.83}{\mathrm{Fe}}_4{\mathrm{Sb}}_{12}$ to $\nu \approx 2.6$ for ${\mathrm{Eu}}_{0.2}{\mathrm{Co}}_4{\mathrm{Sb}}_{12}.$ The transition metal exchange Fe/Co reduces the number of free carriers and causes a crossover of the electronic transport from a hole conductivity regime into electron dominated behavior. Thermoelectric properties change accordingly.