Research at the Laboratory of Crystallography
Charge-order in the low-temperature superstructure of NaV2O5
NaV2O5 crystallizes in the orthorhombic space group Pmmn. Below the transition temperature of 34 K a 2a x 2a x 4c superstructure develops with space group Fmm2. The shifts of the atoms from their positions at room temperature towards the positions in the supercell have been determined by superspace group techniques, and they are indicated by arrows in the figure [1]. Modulated ladders (V2 atoms) and non-modulated ladders (V1 atoms) can be recognized. The superstructure was interpreted in terms of partial charge-order: at room temperature all vanadium atoms are equivalent by symmetry and they are in the mixed valence state 4.5+. Below 34 K, half of the ladders carry a zigzag charge order of 4+ and 5+ Vanadium atoms, while on the non-modulated ladders vanadium remains in the mixed valence state 4.5+ [2].
More recently, it was found that the low-temperature structure is either monoclinic or correspond to a disordered stacking of layers with complete zigzag charge order [3,4].
Literature:
[1] J. Ludecke, A. Jobst, S. van Smaalen, E. Morre, C. Geibel and H.G. Krane (1999): Acentric low-temperature superstructure of NaV2O5. Phys. Rev. Lett. 82, 3633.
[2] S. van Smaalen and J. Ludecke (2000): The valence states of vanadium in the low-temperature superstructure of NaV2O5. Europhys. Lett. 49, 250.
[3] H.Sawa et al., J. Phys. Soc. Jpn. 71, 285-288 (2002).
[4] S. van Smaalen, P. Daniels, L. Palatinus and R. Kremer, Phys. Rev. B 65, 060101 (2002).