Metallic state in the vicinity of molecular orbital crystallization in the d1 thiospinel ZnTi2S4 prepared via a reductive ion-exchange reaction

A novel Ti³⁺-based thiospinel ZnTi₂S₄ is successfully synthesized via a low-temperature ion-exchange reaction. ZnTi₂S₄ shows a signature of metallic ground state evidenced by a contribution of conduction electrons in the heat capacity and Pauli-like paramagnetic susceptibility. These observations contrast to the electronic state of similar Ti³⁺-based spinel MgTi₂O₄ exhibiting the metal-insulator transition associated with a molecular orbital crystallization (MOC). Furthermore, the magnetic susceptibility of ZnTi₂S₄ shows a pseudogap-like behavior indicated by a vast peak in the magnetic susceptibility around 110 K, likely originating from the MOC fluctuation. The origin of the difference in the electronic states of MgTi₂O₄ and ZnTi₂S₄ would be due to the different magnitude of overlap between Ti 3d and p orbitals (O: 2p and S: 3p). The presence of a MOC state in the close vicinity of insulator-metal transition may suggest the importance of itinerancy in a MOC.