title: A reinterpretation of the magnetic properties of the mixed-valence (Nb(V)/Nb(IV)) Zintl phase, Cs(9)Nb(2)As(6).
abstract: A new mixed-valent (Nb(V)/Nb(IV)) Zintl phase, Cs(9)Nb(2)As(6), has been prepared and characterized, recently (Gascoin, F.; Sevov, S. C. Inorg. Chem. 2002, 41, 5920). Niobium is present in the form of isolated, edge-sharing tetrahedral, Nb(2)As(6)(9)(-) dimers. The reported magnetic susceptibility features a broad maximum at approximately 36 K which has been interpreted as the onset of long-range antiferromagnetic order. Such a high transition temperature is difficult to understand as the compound is insulating and the interdimer Nb-Nb distance is 7.2 A. It is shown here that the observed magnetic properties follow straightforwardly from a statistical occupation of the equivalent intradimer Nb sites by equal concentrations of Nb(IV)(4d(1), S = (1)/(2)) and Nb(V)(4d(0)). From this analysis the broad maximum arises from intradimer antiferromagnetic exchange with an exchange constant, J/k = -40 K, and there is no long-range magnetic order except, possibly, below 5 K.
authors.affiliation: Brockhouse Institute for Materials Research and the Department of Chemistry, McMaster University, Hamilton, Canada.
authors.forename: Athena S
authors.initials: AS
authors.lastname: Sefat
authors.forename: J E
authors.initials: JE
authors.lastname: Greedan