J E Greedan | PubMed Health

PubMed2004

Magnetic ordering in the spin-ice candidate Ho2Ru2O7.

Neutron scattering measurements on the spin-ice candidate material Ho2Ru2O7 have revealed two magnetic transitions at T approximately 95 and approximately 1.4 K to long-range ordered states involving the Ru and Ho sublattices, respectively. Between these transitions, the Ho3+ moments form short-ranged ordered spin clusters. The internal field provided by the ordered S=1 Ru4+ moments disrupts the fragile spin-ice state and drives the Ho3+ moments to order. We have directly measured a slight shift in the Ho3+ crystal field levels at 95 K from the Ru ordering.

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PubMed2004

A reinterpretation of the magnetic properties of the mixed-valence (Nb(V)/Nb(IV)) Zintl phase, Cs(9)Nb(2)As(6).

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.

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PubMed2001

Solid-state coordination chemistry: structural influences of copper-phenanthroline subunits on oxovanadium organophosphonate phases. hydrothermal synthesis and structural characterization of the two-dimensional materials [Cu(phen)(VO)(O3PCH2PO3)(H2O)], [[Cu(phen)]2(V2O5)(O3PCH2CH2PO3)], and [[Cu(phen)]2(V3O5)(O3PCH2CH2CH2PO3)2(H2O)] and the three-dimensional phase [[Cu(phen)]2(V3O5)(O3PCH2PO3)2(H2O)].

The hydrothermal reactions of CuCl2*2H2O, Na3VO4, 1,10-phenanthroline, and the appropriate organodiphosphonate ligand yield [Cu(phen)(VO)(O3PCH2PO3)(H2O)] (1), [[Cu(phen)]2(V2O5)(O3PCH2CH2PO3)] (2), [[Cu(phen)]2(V3O5)(O3PCH2CH2CH2PO3)2 (H2O)] (3), and [[Cu(phen)]2(V3O5)(O3PCH2PO3)2(H2O)] (4). Compounds 1-3 exhibit two-dimensional structures. The structures exhibit distinct vanadium building blocks: square pyramidal, mononuclear V(IV) sites in 1, a binuclear unit of corner-sharing V(V) tetrahedra in 2, and a trinuclear unit of corner-sharing V(V) square pyramids and a V(IV) octahedron in 3. The network structures of 1 and 2 are constructed from one-dimensional oxovanadium-diphosphonate chains linked by Cu(II) square pyramids into two-dimensional layers. In contrast, compound 3 exhibits a two-dimensional oxovanadium-organodiphosphonate network, with Cu(II) sites decorating the surfaces. Compound 4 is unique in exhibiting a framework structure, which may be described as a three-dimensional oxovanadium-organodiphosphonate substructure with [Cu(phen)]2+ subunits covalently attached to the surface of channels running parallel to the a-axis. The magnetic properties of 1-4 are also correlated to the structural characteristics. The magnetic behavior of 2 is thus dominated by antiferromagnetic interactions. The magnetic behavior of 1 and 4 is consistent with the presence of two distinct paramagnetic metal ions, Cu(II) and V(IV). In contrast, 3 does not exhibit ferrimagnetic behavior, but rather weak antiferromagnetic coupling. Crystal data: 1, C13H10N2CuP2VO8, monoclinic P2(1)/c, a = 9.0656(5) A, b = 8.6584(5) A, c = 20.934(1) A, beta = 97.306(1) degrees, Z = 4; 2, C26H20N4Cu2P2V2O11, triclinic P1, a = 10.6096(5) A, b = 11.6951(5) A, c = 13.1796(6) A, alpha = 71.369(1) degrees, beta = 70.790(1) degrees, gamma = 80.738(1) degrees, Z = 2; 3, C30H28N4Cu2P4V3O18, triclinic P1, a = 9.4356(6) A, b = 10.6556(6) A, c = 11.0354(7) A, alpha = 118.187(1) degrees, beta = 91.416(1) degrees, gamma = 107.821(1) degrees, Z = 1; 4, C26H20N4Cu2P4V3O18, monoclinic, P2(1)/c a = 8.3947(3) A, b = 16.8401(7) A, c = 11.9144(5) A, beta = 93.903(1) degrees, Z = 2.

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PubMed2001

(Li0.91Mn0.09)Mn2O4.

Lithium manganese oxide crystals with composition (Li(0.91)Mn(0.09))Mn(2)O(4) were synthesized by a flux method. The crystals have a structure closely related to that of the cubic spinel LiMn(2)O(4), but 9% of the lithium ions in the tetrahedral 4a site are substituted by Mn(2+) ions. This substitution lowers the average Mn oxidation state below 3.5+, resulting in a Jahn-Teller distortion of the MnO(6) octahedron.

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