Ni intercalation of titanium diselenide: effect on the lattice, specific heat and magnetic properties

Intercalated Ni_xTiSe₂ compounds with Ni concentration up to x = 0.5 have been investigated using x-ray diffraction, magnetic susceptibility and specific heat measurements. A set of peculiarities caused by the intercalation, such as a significant decrease of the gap between Se–Ti–Se trilayers and depletion of the low-frequency phonons due to the stiffening of the lattice as well as an enhancement of the density of electronic states at low Ni content (x < 0.33), is attributed to hybridization of the Ni 3d states with TiSe₂ bands and formation of covalent-like links between Se–Ti–Se trilayers via the inserted Ni ions. Ni_xTiSe₂ compounds with intercalant content up to x = 0.5 show a paramagnetic behaviour unlike M_xTiSe₂ compounds intercalated by other 3d metals (M = Cr, Mn, Fe, Co). The Ni magnetic moment in Ni_xTiSe₂ is suggested to be of an itinerant nature. A pronounced correlation between the change of the lattice parameter c₀ and the value of the effective magnetic moment, which is observed in Ni_xTiSe₂ as well in M_xTiSe₂ systems intercalated by other 3d metals, indicates that the magnetic moment on the inserted atom is controlled by the hybridization degree of M 3d states with TiSe₂ bands.