Crystal structure and thermoelectric properties of misfit-layered sulfides [Ln₂S₂]_p NbS₂ (Ln = lanthanides)

Polycrystalline samples of misfit-layered sulfides [Ln₂S ₂]_p NbS₂ (Ln = lanthanides) have been prepared using the CS₂ sulfidation method. Except for Ln = Eu and Lu, single-phase samples were prepared. The compounds consist of an alternate stacking of a trigonal prism-type [NbS₂] layer and a double-layered NaCl [rock salt (RS)]-type [Ln₂S₂] block, parallel to the c-axis. Three types of stacking modes of the [NbS₂] layer and the RS-type block have been confirmed with a (3 + 1)-dimensional superspace group description: two face-centered orthorhombic structures and one face-centered monoclinic structure. Slightly cation-deficient samples with Ln = Yb exhibit the highest Seebeck coefficient S ≈ 59 μV/K. With electrical resistivity of ρ = 1.4 m Ω cm and thermal conductivity of κ = 0.69 W/Km, this yields a dimensionless figure of merit of ZT = 0.11 at 300 K.