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

Abstract

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

Charge order induced by electron-lattice interaction inNaV2O5

“…It now appears that the opening of the spin gap may be induced primarily by charge transfer 8,9,10,11 in the low temperature superlattice. 12,13,14,15,16 The discussion on the main driving force of the transition is still going on. It was argued recently that the Coulomb repulsion on an isolated ladder might be too small to cause charge ordering.…”

Spin-gap mode in the charge-ordered phase ofNaV2O5studied by Raman scattering under high pressures

“…21 Sakai et al 41 theoretically studied the selection rule of the ESR absorption from the spin gap by taking the DM interaction into account and they concluded from the experimental result 11 that the DM vector is parallel to the a axis in NaV 2 O 5 . A similar problem for the far-infrared absorption was discussed by Rõõm et al 14 Recently, Sawa et al 4 studied the detailed crystal structure in the charge-ordered state below T c and stated that the structure of C 1/4 phase consists of four inequivalent V sites on the ab plane and the inversion symmetry is broken between the two rungs forming the spin-singlet state. Damascelli et al 31 also stated that a local charge disproportion on each rung of the ladders destroys the inversion symmetry.…”

“…2,8 It was estimated as about 0.3 far below T c . 4,8 The lowtemperature lattice structure ͑C 1/4 phase͒ was first reported to be 2a ϫ 2b ϫ 4c superlattice 3,5 but later Sawa et al 4 determined by a synchrotron-radiation x-ray-diffraction study that C 1/4 phase of this compound was monoclinic with a space group C 2 3 -A112 and the superlattice structure was ͑a − b͒ ϫ 2b ϫ 4c. Ohwada et al 6 determined the stacking pattern along the c axis as AAAЈAЈ by comparing the resonant x-ray scattering results with model calculations.…”

Zone-center phonons inNaV2O5: A comprehensiveab initiostudy including Raman spectra and electron-phonon interaction

“…11 In the lowtemperature phase, ions are displaced from their original positions by distances of the order of 0.05 Å. These displacements have been observed in x-ray diffraction [12][13][14][15][16][17] and can be estimated from infrared 18 and Raman spectroscopy. 19 They stabilize a phase containing zig-zag charge-ordering patterns.…”

“…19 They stabilize a phase containing zig-zag charge-ordering patterns. [14][15][16][17][20][21][22][23] Simultaneously, near T c strong charge fluctuations occur which are induced by dynamical phonons. As a consequence, the spin-spin exchange J is altered.…”