Crystal structures, electronic properties, and pressure-induced superconductivity of the tetrahedral cluster compounds GaNb4S8, GaNb4Se8, and GaTa4Se8

The crystal structures of the tetrahedral cluster compounds GaNb4S8 and GaTa4Se8 were determined by single-crystal X-ray diffraction. They crystallize in the cubic GaMo4S8 structure type (F43m), which can be derived from the spinel type by shifting the metal atoms off the centers of the chalcogen octahedra along [111]. Electrical resistivity and magnetic susceptibility measurements show that the electronic conduction originates from hopping of localized unpaired electrons (S = 1/2) among widely separated Nb-4 or Ta-4 clusters, and thus these materials represent a new class of Mott insulators. Under high pressure we find that GaNb4S8 undergoes a transition from the Mott insulating to a superconducting state with T-c up to 4 K at 23 GPa, similar to GaNb4Se8 and GaTa4Se8. High-pressure single-crystal X-ray studies of GaTa4Se8 reveal that the superconducting transition is connected with a gradual decrease of the octahedral distortion with increasing pressure. DFT band structure calculations show that weakly coupled cluster orbitals are responsible for a high density of states at the Fermi level. The correct insulating magnetic ground state for GaNb4S8 with mu(eff) = 1.73 mu(B) is for the first time achieved by the LDA+U method using U = 6 eV and rhombohedral symmetry.