Possible reappearance of the charge density wave transition in M _xTiSe₂ compounds intercalated with 3d metals

Measurements of the electrical resistivity have been performed on M _xTiSe₂ compounds intercalated with 3d metals (M ≤ Cr, Mn, Fe, Ni) up to x ≤ 0.5. The charge density wave (CDW) transition which is observed in pure TiSe₂ below ∼200K disappears for intercalation x>0.1, while a further increase of the intercalant content (x≥0.25, M ≤ Cr, Mn or Fe) leads to the appearance of pronounced anomalies in the temperature dependences of the resistivity in the same temperature range where the CDW transition takes place in the pure TiSe₂ compound. These anomalies in highly intercalated M_xTiSe₂ are associated with the reappearance of a superstructure formation and associated CDW state. For Mn_0.33TiSe₂, the structural phase transition was evidenced from both powder neutron diffraction measurements and specific heat data. The observed results are discussed in terms of the degree of deformation of Se-Ti-Se sandwiches, which we believe play a key role in the disappearance and reappearance of the CDW state in M_xTiSe₂ as a function of increasing M concentration. The change in the electronic structure due to the intercalation seems not to strongly influence the presence of the CDW.