Synthesis and transport properties of [Ni₃Sn][Ni_4−<i>x</i>S₂], an n-type metal-rich sulfide with an intergrowth structure

By optimizing the synthetic route, a nearly 100% pure Ni₆SnS₂ polycrystalline dense sample was obtained. While the average crystal structure is consistent with the tetragonal <i>I</i>4/<i>mmm</i> symmetry, electron diffraction tomography analysis reveals a lower local symmetry accompanied by weak satellite reflections, indicative of structural modulation at the local scale for this metal-rich layered sulfide. This allowed the Ni₆SnS₂ transport properties to be studied. Despite its metal-like behavior with <i>ρ</i> = 0.16 mΩ cm at 300 K, the material exhibits a moderate thermal conductivity at 250 K (11.4 W m^-1 K^-1). Interestingly, its thermoelectric power factor (PF = <i>S</i>²/<i>ρ</i> = 0.32 mW m^-1 K^-2) exceeds typical values observed in metallic materials.