Structural and electronic properties of the Nb₄-cluster compound Ga₁.₃₃Nb₄X₈ (X = S, Se)

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We report resistivity, thermopower and magnetic susceptibility measurements on the Nb₄-cluster compounds Ga₁.₃₃Nb₄X₈ (X = S, Se), derived from vacancy ordered spinels AₓT₄X₈. The cubic selenide phase is insulating and its resistivity crosses over from ρ T⁻¹ to T⁻¹/² on cooling below 150 K, indicating variable range hopping (VRH) conduction at low temperatures. This is similar to that previously reported for V₄ and Mo₄ cluster compounds. The rhombohedrally distorted sulfide is metallic and shows a minimum in resistance at 56 K below which the resistivity varies as ρ(T) T⁻¹/². The thermopower of the selenide becomes temperature independent below the crossover temperature. These Nb₄ compounds exhibit enhanced Pauli-paramagnetic magnetic susceptibility (χ) irrespective of their transport properties and both undergo a similar transition in χ(T) 30 K. We discuss these properties in the model of hopping conduction under long-range Coulomb repulsion effects and derive consistency between some of the transport and magnetic parameters.