Ni9Sn2S2: An n-type metal-rich 2D sulfide with a metal-to-metal transition

Using spark plasma sintering, a ceramic sample of Ni9Sn2S2 sulfide was densified by starting from a powder synthesized in an evacuated ampoule. The powder x-ray diffraction refinements confirm the formation of the I4/mmm tetragonal Ni9Sn2S2 phase [a = b = 3.6809(2) Å and c = 25.5410(7) Å]. Transmission electron microscopy—imaging and coupled EDX—also reveals the formation of a secondary Ni6SnS2 phase and intergrowth defects in the layer stacking between the Ni9Sn2S2 and Ni6SnS2 phases. More importantly, the structural study demonstrates that Ni9Sn2S2 can be described as the n = 2 term of the generic series of compounds, namely, [Ni3S2][Ni3Sn]n. The Ni3Sn block is of the Cu3Au cubic form. The measurements of the physical properties show metallic behavior, with ρ300K = 0.135 mΩ cm, a residual resistivity ratio RRR = 2.2, and a Seebeck coefficient S300K = −23.5 μV K−1, indicating an n-type feature, the absolute value of which increases with T, as expected for a metal, to reach S650K = −30 μV K−1. This compound also exhibits a large thermal conductivity, κ300K = 8.0 W K−1 m−1, dominated by the electronic part. Interestingly, the small magnetic susceptibility with Pauli-like values shows two transitions at 200 and 125 K, with the first one coinciding to the metal-to-metal transition and to the change of slope in the S(T) curve, suggesting the occurrence of a charge density wave below 200 K. This is discussed in light of the recent developments in topological metals like Ni3In.