Metal−Metal Bonding and Correlated Metallic Behavior in the New Deficient Spinel Ga_0.87Ti₄S₈

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 Ga 0.87 Ti 4 S 8 (space group F −43 m, a = 9.9083(11) Å) 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 11 electrons in known AM 4 X 8 compounds to ∼3 electrons in Ga 0.87 Ti 4 S 8 . XPS measurements show that the valence band of Ga 0.87 Ti 4 S 8 is more dispersed than that of the Mott insulator GaV 4 S 8, 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 0.87 Ti 4 S 8 while a gap appears (0.2 eV) below the Fermi level in the Mott insulator GaV 4 S 8 . The resistivity measured on crystals of Ga 0.87 Ti 4 S 8 showed a metallic behavior with a small residual resistivity ratio. Ga 0.87 Ti 4 S 8 exhibits large electronic specific heat (γ = 58 mJ K −2 mol −1 ) and Pauli susceptibility. All these measurements as well as the Wilson and Kadowaki-Woods ratios demonstrate that Ga 0.87 Ti 4 S 8 exhibits a correlated metal type behavior and is no longer a paramagnetic Mott insulator like other members of the family.