The Magnetic Structure and Electronic Ground States of Mott Insulators GeV₄S₈ and GaV₄S₈

The ground states of magnetic Mott insulators GaV 4 S 8 and GeV 4 S 8 were calculated with full-potential LAPW methods using the LDA + U approach. Both compounds undergo structural distortions from cubic to rhombohedral (GaV 4 S 8 ) or orthorhombic (GeV 4 S 8 ) symmetry at low temperatures. GaV 4 S 8 is ferromagnetic below T C = 10 K, whereas GeV 4 S 8 shows antiferromagnetic order with T N = 13 K. The spin structure of GeV 4 S 8 was determined by neutron diffraction and described in the magnetic space group P b mn 2 1 . The magnetic propagation vector is [ 1 / 2, 1 / 2, 0] relating to the cubic paramagnetic unit cell. The LDA + U calculations ( U = 2 eV) confirm the magnetic insulating ground states for the first time with magnetic moments and energy gaps in very good agreement with the experimental data. This method also reproduces the antiferromagnetic spin ordering of GeV 4 S 8 . The Jahn−Teller instability of the degenerated levels in the V 4 cluster MO drives the structural distortions, depending on the cluster electron count. Our results show how the interplay between the electronic, lattice, and spin degrees of freedom of the V 4 cluster units determines the physical and structural properties of these highly correlated transition-metal sulfides.