Thermoelectric properties of lanthanum sulfide with Sm, Eu, and Yb additives

The Seebeck coefficient and electrical resistivity have been measured for the La3−x Mx S4 alloys with the Th3 P4 -type structure for M=Sm, Eu, Yb, and x=0.1 to 0.9 in order to determine their potential as high-temperature thermoelectric energy conversion materials. An analysis of the lattice parameters of these alloys suggests that Sm has a valence of 2.2 over the entire composition range. Furthermore, the variation of the Seebeck coefficient and electrical resistivity as a function of electron concentration is different for the Sm-doped alloys than for the Eu- and Yb-doped alloys, which are all different from that of the pure binary LaSy alloys. A maximum in the electrical power factor at 1000 °C is found to occur at x ≂ 0.3, but when a reasonable estimate is made of the thermal conductivity the maximum in the figure-of-merit at 1000 °C shifts to larger x values (∼0.7) in all alloy systems. The long term stability of the Sm and Eu ternaries is also reported.