Electronic structure and exchange interactions in BaVS₃

The electronic structure and spin exchange interactions of a quasi-one-dimensional sulfide BaVS₃ have been studied within the density functional theory (DFT) with the generalized gradient approximation (GGA) using the projected augmented wave (PAW) method. The on-site Coulomb repulsion has also been taken into account in the GGA+U approach to unravel the correlation effects on the electronic structure. We found that in the high-temperature hexagonal structure the partially filled t_2g conduction bands including the broad d(3z²−r²) and two narrow e(t_2g) orbitals contribute to the conductivity of BaVS₃, suggesting a weak anisotropy in the electronic transport. Furthermore, our GGA+U band structure calculations predict an insulating ground state with a narrow gap of 0.15 eV for BaVS₃ in the low-temperature (60 K) orthorhombic structure, thereby indicating the Mott-Hubbard mechanism for the low-temperature insulating behavior. Our total energy analyses of the ground-state spin configuration in the low-temperature phases show that the magnetic structure with an intrachain ferromagnetic spin arrangement and an interchain antiferromagnetic spin arrangement that is consistent with a superexchange mechanism through sulfur 3 p orbitals is preferred. We have also estimated the values of exchange integrals by mapping the calculated total energies onto the Heisenberg spin model. It is found that the stronger exchange interaction is ferromagnetic intrachain coupling, while the weaker antiferromagnetic interchain coupling occurs along the a axis. A substantial magnetic anisotropy is found and results in the observed quasi-1D magnetic character in BaVS₃. This suggests the separation of the spin and charge degrees of freedom as a possible cause of the anomalous properties, e.g., the 3D electronic transport and the quasi-1D magnetism of BaVS₃.