Electronic structure α′-NaV₂O₅: Wave-function-based embedded-cluster calculations

Results of ab initio embedded-cluster calculations indicate that the doublet ground state of the V-O_R-V rung originates from a V 3d_xy¹-O_R2p_y¹ -V3d_xy¹ configuration. In the high temperature undistorted geometry the unpaired electron on oxygen is low-spin coupled to the 3d electrons and spin density is equally distributed over the vanadium ions. Based on this picture of the electronic ground state we propose a mechanism for the phase transition at 34 K. We find that a symmetry-broken configuration, R(V_i-O_R) < R(V_j-O_R), leads to V_i3d_xy-O_R2p_y spin singlet formation and stronger V_i-O_R bonding. We suggest that the onset of the phase transition at 34 K is driven by the shift of the bridging rung oxygen towards one of the V neighbors. The calculations predict a reduction of the exchange coupling constant of about 25% when distorting the V-O_R-V rung. At the same time, structural distortions involving the O_L leg oxygens induce alternation of the coupling constant and therewith spin-gap behavior.