Charge-order-driven spin-Peierls transition in α′-NaxV2O5

We conclude from Na-23 and V-51 NMR measurements in alpha(')-NaxV2O5 (x = 0.996) (i) a charge-ordering transition starting at T less than or equal to 37 K and preceding the lattice distortion and (ii) the formation of a spin gap Delta = 106 K at T-e = 34.7 K. Above T-e, only a single Na site is observed in agreement with the Pmmn space group of this first 1/4-filled ladder system. Below T-c = 34.7 K, this line evolves into eight distinct Na-23 quadrupolar split lines, which evidences a lattice distortion with, at least, a doubling of the unit cell in the (a, b) plane. A model for this unique transition, implying both charge density wave and spin-Peierls order, is discussed.