Inversion symmetry in the spin-Peierls compound NaV2O5
  At room-temperature NaV2O5 was found to have the centrosymmetric space group Pmmn. This space group implies the presence of only one kind of V site in contrast with previous reports of the non-centrosymmetric counterpart P21mn. This indicates a non-integer valence state of vanadium.   Furthermore, this symmetry has consequences for the interpretation of the transition at 34 K, which was ascribed to a spin-Peierls transition of one dimensional chains of V4+. 
A. Meetsma (1), J.L. de Boer (1), A. Damascelli (1), J. Jegoudez (2),   A. Revcolevschi(2), and T.T.M. Palstra (1) ((1) Univ...