Intercalation Induced Structural Transition and Magnetic Properties in Ni_xNbS₂

We report the effect of Ni intercalation on the structure and magnetic properties of Ni_xNbS₂. Pristine NbS₂ crystallizes in the rhombohedral 3R phase, which undergoes a structural transition to a mixture of hexagonal (2H) and rhombohedral (3R) phases with Ni intercalation in the range of 0 < x < 0.15 and finally to pure 2H phase in the range of 0.15 ≤ x ≤ 0.25. Further Ni intercalation in NbS₂, in the range of 0.33 ≤ x ≤ 0.4, the 2H phase transforms to an ordered superstructure (Space Group: P6₃22) with lattice parameters of √3a_o × √3a_o × 1c_o. Raman studies shows Ni-S stretching vibration peak at 492.3 cm^-1 suggesting strong bonding of Ni with the host NbS₂ lattice. XPS studies further confirm the charge transfer from Nb to Ni in the Ni intercalated compounds. Theoretical studies show that the 3R phase has lower total energy for pristine NbS₂ while for the intercalated compound, Ni_0.25NbS₂, the 2H phase has lower total energy. Magnetic properties study shows a distinct antiferromagnetic transition at 78 K for Ni_0.33NbS₂, and a kink at 30 K for Ni_0.25NbS₂ suggesting a weak ordering, while no magnetic ordering is observed for Ni_0.15NbS₂.