C. V. Stager

[Cr(bipy)(C2O4)2]−—a versatile building block for the design of heteropolymetallic systems 2. Syntheses and crystal structures of and (AgCr(bipy)(μ-C2O4)2(H2O)2)2 and magnetic properties of the copper(II) derivative

The compounds (1) and (AgCr(bipy)(μ-C2O4)2(H2O)2)2 (2) have been obtained by self-assembling [Cr(bipy)(C2O4)2]− building blocks with Cu(II) and Ag(I) ions, respectively. The molecular and crystal structures of the two compounds were determined. Crystal 1 is monoclinic, , Z = 4 and R = 0.0369. Its structure consists of discrete linear {CrCuCr} trinuclear species with the copper atom lying on an inversion center. The copper atom exhibits [4 + 2] axially elongated octahedral geometry, the apical positions being occupied by two oxalato oxygen atoms. The CuO bond distances are 1.953(4) (H2O), 1.998(3) and 2.357(3) A, the CrO bonds vary from 1.945(3) to 1.982(3) A, and the CrN bonds are 2.031(4) and 2.054(3) A. The Cr…Cu distances are 5.288 A. Crystal 2 is monoclinic, P21/c, a = 14.639(16), b = 15.553(10), , and contains discrete tetranuclear cycles formed by two chromium and two silver atoms bridged by the oxalato groups. The silver atoms are penta-coordinated, exhibiting distorted trigonal bipyramidal geometries. The AgO bond distances vary from 2.350(8) to 2.548(7) A, while the CrO bonds are in the range 1.942(7) to 1.982(6) A. The magnetic susceptibility data of 1 reveal an irregular spin state structure with S = 5/2 ground state and an antiferromagnetic interaction between the central copper(II) and the chromium(III) atoms (J = −18.8 cm−1.

Crystal structure, short range and long range magnetic ordering in CuSb2O6

CuSb2O6 crystallizes in a monoclinically distorted trirutile structure. Atomic positions were determined by profile refinement of neutron powder diffraction data, space group P21n, a = 4.6349(1), b = 4.6370(1), c = 9.2931(1), β = 91.124(2). Magnetic susceptibility data exhibit a broad maximum at about 60 K and an abrupt transition at 8.5 K. The high temperature data can be fitted to a Curie-Weiss Law giving μeff = 1.758 and θ = −48 K. Although the crystal structure indicates a nearly square planar Cu2+ lattice as in other trirutiles, a CuOOCu superexchange pathway seems to be dominant, giving rise to short range correlations which are approximately one dimensional. The high-temperature susceptibility is explained well by the 1-d Heisenberg model with Jk = −43.1 K. Analysis by Oguchis method gives a ratio of interchain to intrachain coupling constants of about 2 × 10−3.

Fabrication of textured Bi-Sr-Ca-Cu-O thick film by electrophoretic deposition

Bi‐Sr‐Ca‐Cu‐O thick films (≊5–40 μm) were electrophoretically deposited on silver foil using ethyl alcohol as the suspending liquid. The deposition voltage was ≊70 V/cm and the current ≊550 μA/cm2. X‐ray diffraction of the as‐deposited sample shows texturing. This texturing was enhanced by pressing the film uniaxially at ≊82 MPa. Samples were sintered at 810 °C in a 10% oxygen‐in‐argon atmosphere. Liquid at the sintering temperature promotes textured grain growth. The critical current density (Jc) of the film determined by magnetic measurement at 5 K in a 2 kOe field is ≊1.3×104 A/cm2.

Synthesis and magnetic properties of a new oxalato-bridged heterotrinuclear complex, [NiCr2(bipy)2(C2O4)4(H2O)2]·H2O. A rare case of antiferromagnetic coupling between Cr(III) and Ni(II) ions

A new heterotrinuclear complex, [NiCr2(bipy)2(C2O44(H2O)2]·H2O, has been synthesized by self-assembling [Cr(bipy)C2O4)2]− building blocks with Ni(II) ions. The synthesis war consists of the reaction of a barium(II) precursor, [BaCr2(bipy)2(C2O4)4(H2O)]·H2O, with nickel(II) sulfate. Variable-temperature magnetic susceptibility measurements indicate an antiferromagnetic coupling between Ni(II) and Cr(III) ions (J  − 0.6cm−1, DNi  0.4 cm−1,H = −J(SCrlSNi+SCr2SNi)+SNiDNiSNi).

Short-range ordering in a three-dimensionally frustrated magnet, Tb2Mo2O7, by wide- and small-angle neutron diffraction

Tb2Mo2O7 is an ordered crystalline oxide with the pyrochlore structure. This material undergoes a magnetic transition at 25 K which is similar to that of a spin glass. Neutron diffraction data indicate the presence of short range magnetic order via broad, diffuse reflections which persist above 25 K. Below 25 K the diffuse peaks increase in intensity but do not narrow. Fourier analysis of the data yield an approximation to the radial distribution function which is interpreted in terms of spin–spin correlations up to fourth neighbors. The first, third, and fourth neighbor interactions are ferromagnetic but the second neighbor interaction is strongly antiferromagnetic and involves only a Tb‐Mo coupling. SANS data show magnetic scattering only at large Q values, reenforcing the view that antiferromagnetic interactions dominate. The Q‐integrated magnetic SANS intensities show a temperature dependence similar to that for the correlation functions.

ND4FeBr3, a new one‐dimensional S=1 antiferromagnet

Compounds of chemical formula AFeX3 (A=Rb,Cs,Tl, and X=Cl, Br) provide an isomorphous series of one‐dimensional induced‐moment magnets with an effective spin S=1. They display a variety of ordered magnetic structures at low temperatures, depending on the sign and relative magnitudes of the intra‐ and interchain exchange and any magnetic dipole–dipole interactions. We report the first preparation of the salt NH4FeBr3. Powder neutron diffraction measurements on a perdeuterated sample showed that at room temperature the unit cell is hexagonal with cell parameters a=7.398(3) A and c=6.331(2) A. On cooling, structural phase transitions to cells of successively lower symmetry were observed. These are probably caused by cooperative reorientations of the ND+4 tetrahedra. At 4.8(2) K and 2.8(2) K additional Bragg peaks were observed. These additional reflections were attributed to magnetic long‐range order with antiferromagnetic exchange both in the basal plane and along the c axis and could be indexed on an ortho...

Crystal structure and short‐range and long‐range order in CuSb2O6 (abstract)

CuSb2O6 crystallizes in a monoclinically distorted trirutile structure. Atomic positions were determined by profile refinement of neutron powder diffraction data. An important feature of the structure is the presence of nearly square planar Cu2+ layers. The Cu‐O‐Cu superexchange pathways are such that short‐range interactions are approximately one‐dimensional. Evidence for short‐range order is found from a susceptibility maximum at 60 K. Long‐range order occurs near 25 K and an additional, first‐order transition occurs at 8 K.

Superconductivity of the Nb3As system

We have found that the metastable A‐15 Nb3As phase is present together with the more stable Ti3P‐type Nb3As in sintered samples. The superconducting transition temperature for the A‐15 phase is 6.5 K and the upper critical field at 0 K is 1.82 T.