Iron Aluminide Fe₂Al₅ has a rigid framework of both fully occupied Al and Fe sites and chains of partially occupied Al sites. On the other hand, Fe₄Al₁₃ possesses a large unit cell with 102 atoms. These complex and peculiar crystal structures bring a low lattice thermal conductivity. Here, we report the thermoelectric properties and discuss how the chain structure and large unit cell can lead to a low lattice thermal conductivity.
   The calculated room-temperature lattice thermal conductivity by using the Wiedemann-Franz law is approximately 1.5 W/mK and 0.8 W/mK for Fe₂Al₅ and Fe₄Al₁₃, respectively. From the comparison with other Fe-Al alloys, which have neither plural partially occupied sites nor a large unit cell, we found that (1) the heat capacity does not decrease at high temperature, i.e. Al atoms at partially occupied sites do not behave as liquid, (2) the speed of sound for Fe₂Al₅ and Fe₄Al₁₃ is almost identical among Fe-Al alloys, i.e. the average phonon group velocity of acoustic modes for Fe₂Al₅ and Fe₄Al₁₃ is not slower than that of Fe-Al alloys, (3) the electrical conductivities of Fe₂Al₅ and Fe₄Al₁₃ are lower than those of the other Fe-Al alloys. These results suggest that the low lattice thermal and electrical conductivities are brought by short relaxation times of both phonons and electrons due to chemical disorder such as the partially occupied sites. In particular, the unit cell of the Fe₄Al₁₃ includes 102 atoms, which beneficially reduce the lattice thermal conductivity. The maximum dimensionless figure of merit is approximately 0.02 for both Fe₂Al₅ and Fe₄Al₁₃ at 973 K.