Spin-singlet clusters in the ladder compound NaV₂O₅

The space group of α′−NaV₂O₅ turns below T_c = 34K from Pmmn with all V sites equivalent, into Fmm 2 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⁺, V^4.5+, and V⁵⁺. The singlet formation is not associated with dimerization on the spin ladder, but with the formation of spin clusters. Finally, we ascribe the quadrupling of the c axis to the large polarizability of the V₂O₅ skeleton.