Electronic and Thermal Properties of Crystalline and Polycrystalline Ni3S2: A Metal with Weak Grain Boundaries Contribution

Ni3S2 is a metallic nickel sulfide with promising catalytic properties, yet its transport behavior remains poorly investigated. In this work, we synthesize single-crystalline (via CVT) and densified (via SPS) polycrystalline Ni3S2 samples and characterize their electronic and thermal properties over a broad temperature range, extending up to 700 K for the polycrystalline material. Both samples exhibit metallic conductivity with electron-like carriers, as indicated by a negative Seebeck coefficient (−2.5 μV·K−1 at 320 K) and Hall effect measurements, which yield an apparent carrier density of 4.0 × 1022 cm−3 within a single-band approximation. Polycrystals show a substantially higher residual resistivity ratio (RRR ∼50) than that of single crystals (RRR ∼14). Moreover, the polycrystalline material exhibits a higher thermal conductivity peak at 50 K, indicating that grain boundaries do not strongly limit thermal transport in the highly densified polycrystalline sample and that additional growth-induced structural disorder in the CVT-grown single crystals contributes to phonon scattering. Magnetoresistance reaches +20% at 9 T and 12 K for single crystals, decreasing to +2% at 100 K, with the polycrystalline sample exhibiting even stronger low-temperature responses, further highlighting the limited influence of grain boundaries on charge transport in the highly densified materials. High-temperature measurements confirm the persistence of metallic transport, with ρ reaching 50 μΩ.cm at 700 K. By comparing single-crystalline and polycrystalline Ni3S2, this work establishes a reference for its intrinsic transport behavior and provides insights relevant to researchers seeking to understand and tailor the role of grain boundaries and other forms of disorder in catalytic applications.