Synthesis, Characterization, and Electronic Properties of Nickel-Based Pnictides Th ₂ Ni ₅ As ₃ N ₂

We report the synthesis, structure, and physical properties of a novel quaternary compound, Th₂Ni₅As₃N₂. The target compound crystallizes in a tetragonal structure with lattice parameters <i>a</i> = 4.0650(4) Å and <i>c</i> = 22.0728(5) Å at 300 K. Its structure can be described as alternating stacking of antifluorite-type double-layered [Ni₅As₃] subunits and fluorite-type [Th₂N₂] layers along the <i>c</i> axis. The reduced axial ratio of the compound is attributed to the built-in chemical pressure exerted by the [Th₂N₂] layers. A structural phase transition, accompanied by a resistivity anomaly and a specific heat jump, was observed at around 280 K. No superconductivity was detected down to 0.4 K. Electrical resistivity, heat capacity, magnetic susceptibility measurements, and first-principles calculations suggest metallic behavior with Pauli paramagnetism. Drawing from both experimental data and band calculations, the structural transition in relation to chemical pressure and the [Ni₅As₃] subunit is discussed.