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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...

The low-temperature (LT) superstructure of α ′ -NaV2O5 was determined by synchrotron radiation x-ray diffraction. Below the phase transition temperature associated with atomic displacement and charge ordering at 34K, we observed the Bragg peak splittings, which evidence that the LT structure is monoclinic. It was determined that the LT structure is (a − b) × 2b × 4c with the space group A112 where a, b and c represent the high temperature orthorhombic unit cell. The valence estimation of V ions according to the bond valence sum method shows that the V sites are clearly separated into two groups of V 4+ and V 5+ with a zigzag charge ordering pattern. This LT structure is consistent with resonant x-ray and NMR measurements, and strikingly contrasts to the LT structure previously reported, which includes V 4.5+ sites. 71.30+h,71.27+a,61.10Eq