Coupled Structural and Electronic Transitions in the Layered Ni _5.7 SnSe ₂ Metal

Ni 5.7 SnSe 2, the first member ( n = 1) of the layered intergrowth series [Ni 3 Sn] n [Ni 4−δ Se 2 ] (δ = 1.3), was synthesized as a dense, single-phase polycrystalline material by optimizing the Ni content through solid–state reaction followed by spark plasma sintering. At room temperature, laboratory powder X-ray diffraction can be attributed to an average tetragonal structure (space group I 4/ mmm ), in agreement with previous reports; however, weak superstructure reflections reveal a deviation from this high-symmetry average description. Through a combined analysis using single-crystal X-ray diffraction, electron microscopy and synchrotron powder diffraction, the structure is unambiguously resolved in monoclinic symmetry, revealing long-range ordering between the occupied and empty Ni sites within the [Ni 4−δ Se 2 ] layers. Variable-temperature synchrotron powder diffraction demonstrates that the superstructure reflections and the associated monoclinic peak splitting vanish reversibly above 490 K, indicating an order–disorder structural transition. Electron diffraction and high-angle annular dark-field scanning transmission electron microscopy further confirm the monoclinic symmetry at room temperature, and reveal the presence of stacking faults along the stacking direction of the layers. Electrical transport measurements indicate metallic behavior (ρ = 0.08 mΩ cm at 300 K, RRR ≈ 4.5) and reveal a sharp, reversible anomaly in both resistivity and Seebeck coefficient at the transition temperature, establishing a strong coupling between Ni ordering and electronic transport.