Charge-order-driven spin-peierls transition in alpha(')-NaxV2O5

ABSTRACT

We conclude from 23Na and 51V NMR measurements in alpha(')-NaxV2O5 (x = 0.996) (i) a charge-ordering transition starting at TDelta = 106 K at T(c) = 34.7 K. Above T(c), 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 23Na 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.