Low-Temperature Structure of the Quarter-Filled Ladder Compound α'-NaV₂O₅

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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 34 K, we observed Bragg peak splittings, which provide 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 V⁴⁺ and V⁵⁺ with a zigzag charge-ordering pattern. This LT structure is consistent with resonant X-ray and NMR measurements, and is in striking contrast to the LT structure previously reported, which includes V^4.5+ sites.