Structural and magnetic transitions in the Mott insulator GaNb4S8

Abstract

GaNb₄S₈ is an unconventional Mott insulator where electronic conduction takes place by hopping of localized carriers (S = ½) between widely separated tetrahedral Nb₄ clusters. GaNb₄S₈ is cubic (space group F3m) and paramagnetic (μ_eff = 1.7 μ_B f.u.⁻¹, θ_CW = −298 K) between r. t. and 31 K, where the magnetic moment drops abruptly. Specific heat measurements indicate a phase transition at T_t = 31 K with a small enthalpy change ΔH_t ≅ 130 J mol⁻¹ and a linear coefficient γ = 19 mJ mol⁻¹ K⁻² below 10 K. According to single crystal X-ray diffraction data, GaNb₄S₈ is tetragonal below T_t [P2₁m, a = 9.992(1) Å, c = 9.978(2) Å] where the Nb₄ cluster units distort slightly. The cluster elongations are oriented along the directions [11], [11], [11], []. Assuming the spin magnetic moments are in alignment with the deformation directions of the clusters, a non-collinear antiferromagnetic spin structure emerges and all spin components compensate to zero. The structural transition can explain the simultaneously occuring drop of the magnetic moment. LDA + U band structure calculations support this model and reveal that the magnetic insulating ground state is in good agreement with the experimental results.