Low-temperature superstructure and charge-ordering effect inη−Na1.286V2

F. Duc

¹

,

P. Millet

²

,

S. Ravy

³

et al.

Abstract

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 4.5ϩ. We show that these two sites are occupied by V 4ϩ and V 5ϩ , 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.

Low-temperature behavior of the ESR linewidth in a system with a spin gap:η−Na1.286V2O5

“…1 is similar to that measured by SQUID magnetometry and which we reported in Ref. 12: in particular, we note the small kink just above 100 K associated with the occurrence of the second-order phase transition mentioned above, as well as the decrease towards zero below this temperature indicating the presence of the spin gap ͑the low-temperature increase below 7 K is attributed to impurities, as we explained in Ref. 12͒.…”

“…We attribute this behavior to the magnetic correlations accompanying the second-order phase transition reported in Ref. 12 and which we mentioned above. The discrepancy between the two curves as regards the presence or the absence of the 80 K peak would then be directly related to the fact that the phase transition consists basically of a dimerization along the y direction of the structure.…”

“…The origin of the spin gap can hence be related to the formation of this superstructure below 100 K in the system. Preliminary X-band electron-spin resonance ͑ESR͒ and complementary magnetic susceptibility data, which we reported recently, 12 give additional evidence for the transition around 100 K and confirm the existence of the spin gap, which we have estimated to be of the order of 35 K.…”

“…This interpretation, however, remains hard to reconcile with the fact that each magnetic unit contains, as mentioned above, nine spins 1 2 . Very recently, low-temperature structural measurements 12 have revealed the existence, around 100 K, of a structural second-order phase transition in this compound, associated with a concomitant charge ordering. The low-temperature structure remains centrosymmetric with space group P2/c but with a doubled b lattice parameter in comparison with that at high temperature, and the two vanadium sites which host V 4.5+ ions at room temperature become stabilized as one V 4+ and one V 5+ at low temperature.…”

“…The static magnetic field, H, was applied in both directions, parallel and perpendicular to the plane containing the double strings and to which we shall refer as the ͑x , z͒ plane of the structure, following the notation of Ref. 12.…”