Effect of La ³⁺ substitution on the structural and thermoelectric properties of Ca _{3- x} La _x Co ₄ O _{9+ δ}

The thermoelectric properties of Ca ₃ Co ₄ O ₉ were optimized by the substitution of La ³⁺ for Ca ²⁺ in Ca ₃ Co ₄ O ₉ . The La ³⁺ substitution significantly enhanced the thermoelectric power factor and reduced the lattice thermal conductivity. The lattice thermal conductivities at 800 °C for x = 0 and 0.3 samples were 1.80 and 1.34 Wm ⁻¹ K ⁻¹ , respectively. The reduced thermal conductivity was mainly attributed to mass and strain field fluctuations in the crystal lattice. Ca _2.7 La _0.3 Co ₄ O _{9+ δ} showed the largest dimensionless figure-of-merit (0.282 at 800 °C) by combining high power factor and the lowest lattice thermal conductivity. This work demonstrates that the La ³⁺ substitution is a highly effective approach for improving high-temperature thermoelectric properties.