Glass-like Lattice Thermal Conductivity and Thermoelectric Properties of Incommensurate Chimney-Ladder Compound FeGe_γ

We have characterized the thermoelectric properties of FeGe γ, which is one of the promising Nowotny chimney-ladder compounds. A glass-like low lattice thermal conductivity below 1.0 W m –1 K –1 is observed around 373 K because of its complex structural nature, i.e., incommensurate structure. A first-principles band structure calculation implies that a narrow band gap of ∼0.2 eV is formed near the Fermi level for a hypothetical composition of Fe 2 Ge 3 with a Ru 2 Sn 3 -type structure, leading to a large power factor of 1.90 mW m –1 K –2 near 600 K. The maximum dimensionless figure-of-merit of 0.57 is attractive as a starting point; calculation using the Boltzmann transport equation under a constant relaxation time approximation predicts that a further enhancement of ZT exceeding unity at 600–700 K can be achieved by optimizing the valence electron count per transition metal and further reduction of the lattice thermal conductivity.