Metal-metal bonding and correlated metallic behavior in the new deficient spinel Ga0.87Ti4S8

We have synthesized and characterized the first Ti member of the AM(4)X(8) (A = Ga, Ge; M = V, Nb, Ta, Mo, X = S, Se) family of compounds that exhibit a deficient spinel structure with tetrahedral transition metal clusters. Single-crystal X-ray structure determination of Ga0.87Ti4S8 (space group F-43m, a = 9.9083(l 1) A) revealed the presence of Ga vacancies and a partial declustering of the transition metal atoms compared to other members of the family. This declustering is likely associated to the decrease of the electronic filling of the clusters from 7 to I I electrons in known AM4X8 compounds to similar to 3 electrons in Gao.87Ti4S8- XPS measurements show that the valence band of Ga0.87Ti4S8 is more dispersed than that of the Mott insulator GaV4S8, which suggests a strengthening of the transfer integrals induced by the slackening of the transition metal clusters. A Fermi step consistent with a metallic character is observed in Ga-0.87Ti4S8 while a gap appears (0.2 eV) below the Fermi level in the Mott insulator GaV4S8. The resistivity measured on crystals of Ga0.87Ti4S8 showed a metallic behavior with a small residual resistivity ratio. Ga0.87Ti4S8 exhibits large electronic specific heat (gamma = 58 mJ K-2 mol(-1)) and Pauli susceptibility. All these measurements as well as the Wilson and Kadowaki-Woods ratios demonstrate that Ga0.87Ti4S8 exhibits a correlated metal type behavior and is no longer a paramagnetic Mott insulator like other members of the family.