Thermoelectric properties of quasi-skutterudite single crystals M 5Rh6Sn18 ( M = Sc, Y, Lu)

We present a comparative study of the thermoelectric (TE) properties of single-crystalline quasi-skutterudites M5Rh6Sn18 (M = Sc, Y, Lu), based on temperature-dependent in-plane measurements of electrical resistivity ρ(T), Seebeck coefficient S(T), and thermal conductivity κ(T). X-ray diffraction analysis confirms a tetragonal structure and suggests the presence of structural disorder. The ρ(T) data reveal poor metallic behavior for M = Sc, nonmetallic-like behavior for M = Y, and intermediate characteristics for M = Lu. The nonmetallic-like ρ(T) in M = Y and Lu can be phenomenologically described by the Mott variable-range hopping model over limited temperature ranges, suggesting disorder-induced carrier localization. Negative S(T) and its nearly linear high-temperature variation indicate electron-dominated diffusive TE transport. The κ(T) data reveal a balance between electronic and lattice contributions, with the lattice κ remaining low (≤2 W m−1 K−1 at 300 K). The modified Debye–Callaway analysis of the lattice component indicates significant contributions from point-defect scattering and effective scattering associated with internal structural disorder. Despite the low lattice thermal conductivity, the overall TE performance is modest, with a maximum power factor (PF) of ∼0.9 μW cm−1 K−2 and a figure of merit (ZT) ∼0.005 at 300 K for M = Lu. These findings highlight the impact of disorder and defects on both charge and phonon transport in this material family.