Zigzag Charge Ordering in α^′-NaV ₂O ₅

Abstract

23 Na NMR spectrum measurements in α ′ -NaV2O5 with a single-crystalline sample are reported. In the charge-ordered phase, the number of inequivalent Na sites observed is more than that expected from the low-temperature structures of space group Fmm2 reported so far. This disagreement indicates that the real structure including both atomic displacement and charge disproportionation is of lower symmetry. It is suggested that zigzag ordering is the most probable. The temperature variation of the NMR spectra near the transition temperature is incompatible with that of second-order transitions. It is thus concluded that the charge ordering transition is first-order.KEYWORDS: α ′ -NaV 2 O 5 , NMR, charge ordering, crystal structure, spin-ladder, trellis latticeSince the phase transition into a spin-gapped phase in α ′ -NaV 2 O 5 was reported, 1) a lot of experimental efforts have been devoted to understand the nature of this transition. Although it was initially identified as a spin-Peierls transition, a recent room-temperature structural study 2) questioned this interpretation: it concluded that all V ions are in a uniform oxidation state of V 4.5+and form a quarter-filled trellis lattice composed of twoleg ladders. After that, 51 V NMR measurements 5) revealed charge ordering of V 4+ and V 5+ states below the transition temperature T C ∼ 34 K. Subsequent theoretical studies showed that long-range Coulomb interaction can induce charge ordering in a quarter-filled trellis lattice. 6-9) These studies suggest zigzag or linear chain ordering depending on the strength of the longrange Coulomb interactions. The proposed mechanism of charge ordering is similar to that for charge density wave in quarter-filled systems of low-dimensional organic compounds, 10) suggesting some common physics to α ′ -NaV 2 O 5 and these systems. Soon after the finding of the transition, an x-ray diffraction measurement revealed superlattice formation of 2a × 2b × 4c in the charge-ordered phase, 11) but the detailed low-temperature structure has been unknown yet. Recently, two x-ray diffraction studies of the lowtemperature structure were reported. These indicate almost the same structure of space group Fmm2 , but their assignments of V electronic states are different. One suggests a structure consists of half-filled (V 4+ ) and empty (V 5+ ) ladders. 12) This charge distribution disagrees with a recent x-ray anomalous scattering measurement, which indicates charge modulation along b axis.13) The other suggests a structure including three different electronic state of V 4+ , V 5+ and V 4.5+ . 14) This structure is incom- * E-mail: ohama@physics.s.chiba-u.ac.jp patible with the 51 V NMR measurement, 5) which clearly shows that all the V sites split into two groups of V 4+and V 5+ states and that no V sites remains to be V 4.5+ . Thus, the low-temperature structure and the charge ordering pattern are still under discussion.In this letter, we report 23 Na NMR spectrum measurements with a single-crystalline sample. The obtained NMR spectra in the c...

Orthorhombic versus monoclinic symmetry of the charge-ordered state ofNaV2O5

“…There are eight crystallographically independent vanadium atoms, but there are only four independent sodium atoms. Despite the good fit to the diffraction data, this model is not in agreement with the observations made by NMR [20,21].…”

“…Most notably, 23 Na NMR has found eight resonances, that were interpreted as being due to eight crystallographically independent atomic sites, whereas the crystal structure in Fmm2 only has six indepent Na sites. It was proposed that the true symmetry of the low-temperature structure might be a subgroup of Fmm2 corresponding to the loss of the F-center [21]. Alternatively, monoclinic symmetry was considered [19].…”

“…A monoclinic distortion is ruled out, while the ordered structure with triclinic symmetry did not explain the NMR data of Refs. [20,21]. In order to accommodate observations by experimental techniques other than X-ray scattering, we propose that the true superstructure might be composed of layers with zigzag charge order on all ladders, that shows stacking disorder within a superstructure of orthorhombic symmetry, e.g.…”

Dzyaloshinskii-Moriya interaction inNaV2O5:A microscopic study

“…11 A series of recent studies has addressed the nature of the lowtemperature state [12][13][14][15][16][17] . In this context it is of interest that very recent Electron Spin Resonance (ESR) experiments 18,19 have detected a considerable anisotropy of the absorption intensity with respect to the magnetic field orientation, which has been attributed to the DM interaction.…”

Lattice vibrations ofα′−NaV2O5Popova1, Sushkov2, Климин3 et al. 2002Phys. Rev. B215120View full textAdd to dashboardCite

“…More recent bond-valence calculations suggest, however, only two vanadium valences despite the three crystallographically inequivalent V sites [15]. Nevertheless, several observations, in particular, the properties of X-ray anomalous scattering [13], can be better explained if one assumes the modulation of all the vanadium ladders, and therefore different subgroups of F mm2 were considered [17,18]. In their Raman scattering study of folded modes, the authors of Ref.…”