Structural and magnetic transitions in the Mott insulator GaNb4S8

GaNb4S8 is an unconventional Mott insulator where electronic conduction takes place by hopping of localized carriers ( S = 1/2) between widely separated tetrahedral Nb-4 clusters. GaNb4S8 is cubic ( space group F (4) over bar 3m) and paramagnetic (mu(eff) = 1.7 mu(B) f. u.(-1), theta(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 Delta H-t congruent to 130 J mol(-1) and a linear coefficient gamma = 19 mJ mol(-1) K-2 below 10 K. According to single crystal X-ray diffraction data, GaNb4S8 is tetragonal below T-t [P (4) over bar2(1)m, a = 9.992( 1) angstrom, c = 9.978(2) angstrom] where the Nb-4 cluster units distort slightly. The cluster elongations are oriented along the directions [(1) over bar 11], [ 1 (1) over bar1], [ 11 (1) over bar], [(1) over bar(1) over bar(1) over bar]. 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.