Thermoelectric properties of double-filled skutterudites Ca _1-xla_xFe_4-yCo_ySb₁₂

To study the effect of double filling of the skutterudite iron antimonides on the thermal conductivity, we have synthesized dense samples of Ca _1-xLa_xFe₄Sb₁₂ (0 ≤ × ≤ 1) and Ca_0.3La_0.6Fe_4-yCO_ySb ₁₂ (y = 1, 1.5) by the spark plasma sintering technique. Electron-probe microanalysis revealed that the double-filled samples with x = 0.25, 0.5, and 0.75 consist of three phases of micrometer scale; CaFe ₄Sb₁₂, Ca-rich phase, and La-rich phase, respectively. The lattice thermal conductivity at 150 K is smaller by 10-20% than that for the end compounds. This is attributable to the scattering of heat conducting phonons by the local fluctuations in both mass and charge of Ca²⁺ and La³⁺ ions. In the full range of x from 0 to 1, the thermopower at 300 K stays at 70 μV/K. This value was increased to 120 μV/K by partial substitution of Co for Fe. The dimensionless figure of merit ZT for Ca _0.3La_0.6Fe₃CoSb₁₂ is estimated to be 0.9 at 800 K, which is comparable to the value reported for p-type Ce _0.44Yb_0.32Fe₃CoSb₁₂.