title: Spin-singlet clusters in the ladder compound NaV2O5
abstract: 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+. 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.
authors.affiliation: Solid State Chemistry Laboratory, Materials Science Center, University of Groningen, 9747 AG Groningen, The Netherlands.
authors.lastname: de Boer JL
authors.initials: A
authors.lastname: Meetsma
authors.initials: J
authors.lastname: Baas
authors.initials: TT
authors.lastname: Palstra