Spin-singlet clusters in the ladder compound NaV2O5

The space group of alpha'-NaV2O5 turns below T-c = 34 K from Pmmn with all V sites equivalent, into Fmm2 with three independent vanadium sites per layer. This is incompatible with models of charge ordering into V4+ and V5+. Our structure determination indicates that the phase transition consists of a charge ordering with three distinct valence states, formally V4+, V4.5+, and V5+. Th, singlet formation is not associated with dimerization on the spin ladder, but with the formation of spin clusters. Finally, wt: ascribe the quadrupling of the c axis to the large polarizability of the V2O5 skeleton.