Low-temperature superstructure and charge-ordering effect in η-Na1.286V2O5

In this work, we report on x-ray diffraction, electron spin resonance, and magnetic susceptibility measurements performed on-Na 1.286 V 2 O 5. The x-ray data clearly reveal the existence, around 100 K, of a structural second-order phase transition, stabilizing a superstructure associated with charge ordering. The complete superstructure is determined from x-ray scattering data measured at 15 and 40 K. The low-temperature structure remains centrosymmetric with space group P2/c but with a doubled b lattice parameter. An analysis using superspace symmetry shows that one of the vanadium atoms and its apical oxygen, namely the V7 and O7 atoms, are the most displaced. Two sites for V7 atoms are identified below the transition instead of one with the formal valence of V 4.5. We show that these two sites are occupied by V 4 and V 5 , which corresponds to a charge ordering. We suggest that this transition is driven by electronic repulsions. The magnetic measurements give additional evidence for this transition and for the opening of a spin gap at low temperature. This is discussed in conjunction with superstructure formation.