Abstract
The ground states of magnetic Mott insulators GaV4S8 and GeV4S8 were calculated with full-potential LAPW methods using the LDA + U approach. Both compounds undergo structural distortions from cubic to rhombohedral (GaV4S8) or orthorhombic (GeV4S8) symmetry at low temperatures. GaV4S8 is ferromagnetic below TC = 10 K, whereas GeV4S8 shows antiferromagnetic order with TN = 13 K. The spin structure of GeV4S8 was determined by neutron diffraction and described in the magnetic space group Pbmn21. 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 GeV4S8. The Jahn−Teller instability of the degenerated levels in the V4 cluster MO drives the structural distortions, depending on the cluster electron count. Our results ...
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