O. Peña

Crystal structures and luminescent properties of terbium(III) carboxylates

Abstract Single crystals of three terbium(III) carboxylates of formulae [Tb2(CH3COO)6(H2O)4]·4H2O (1), [Tb2(CF3COO)6(H2O)6] (2) and [Tb(Hoda)3]·H2oda·H2O (3) (H2oda=2,2′-oxydiacetic acid) were obtained and their structures determined by X-ray crystallography. Compounds 1 and 2 are dimeric, in the former the terbium atoms are bound by two tridentate carboxylates in the μ2-bridging mode, whereas in the latter the bridging is fourfold with all carboxylates in the syn–syn coordination mode. Compound 3 is mononuclear containing three tridentate Hoda anions, and consecutive units are linked by a network of H-bonds involving the interstitial molecules. The luminescence spectra of the carboxylates were analyzed in the solid state and in aqueous solution. Comparison of the emission lifetimes in H2O and D2O allowed the determination of the average value for q, the number of coordinated water molecules, being 9.2 for 1 and 2 and 3.6 for 3, respectively. The quenching effect of Cu(II) on the luminescence of the terbium(III) carboxylates was evaluated through the emission decay constants. From the addition of Cu(II) to an aqueous solution of 3, single crystals of polymeric [{Cu3Tb2(oda)6(H2O)6}·12H2O]n (4) were isolated with completely quenched luminescence. Compound 4 exhibits an overall antiferromagnetic interaction.

Spin reversal and ferrimagnetism in (Gd,Ca)MnO3

The gadolinium-based manganite GdMnO3, partially substituted by calcium at the cationic site, has been investigated through its crystallochemical and magnetic properties. The Gd1−xCaxMnO3 solid solution crystallises in an orthorhombic perovskite-type structure (S.G. Pbnm) in the range 0 ≤ x ≤ 0.45. With increasing x, the structure evolves towards a quasi-tetragonal symmetry, with c/√2 ≈ a < b, while the orthorhombicity factor b/a decreases. The magnetisation in the ordered state was studied as a function of temperature and applied field. ZFC + FC cycles show that the solid solution can be described as a ferrimagnetic-like system, in which negatively-polarised gadolinium moments behave as free spins under the internal field of the ordered Mn sublattice. As a result, the spontaneous magnetisation changes sign upon cooling (FC process), reaching similar values as those obtained upon reversal of the magnetic field. Results are explained in terms of two interacting magnetic sublattices: a Mn-based ferromagnetic one and a negatively-aligned gadolinium network. The local field at a given site depends on the exchange interaction between these two sublattices, leading to a spin reversal when the magnetic moment of the gadolinium sublattice is larger than the ferromagnetic network. This interpretation is confirmed through the thermal evolution of the remanent magnetisation Mrem(T) after freezing the spins using a high magnetic field.

Synthesis, X-ray crystal structures and magnetic properties of Cu(II)(NITpPy)2[N(CN)2]2·solv (NITpPy=nitronyl nitroxide radical, solv=H2O or CH3CN). From discrete molecules to 2-D polymeric coordination compounds

Abstract The synthesis, X-ray crystal structures and magnetic properties of five new copper(II) complexes containing nitroxide radicals Cu(NITpPy)2(NO3)2 (1), Cu(NITpPy)2(CH3COO)2 (2), Cu(NITpPy)2[N(CN)2]2·(H2O)2 (3), Cu(NITpPy)2[N(CN)2]2·3CH3CN (4) and Cu(NITpPy)2[N(CN)2]2·2CH3CN (5) are reported. Compounds 1–3 consists of discrete molecules in the solid state, but in compounds 4 and 5 the Cu(NITpPy)22+ units are connected through N(CN)2− bridging ligands in μ2 coordination developing, respectively, into one-dimensional and two-dimensional polymeric networks. For all compounds, the magnetic properties have been investigated and antiferromagnetic interactions dominate at low temperature.

Rare earth based chevrel phases REMo6X8: Crystal growth, physical and superconducting properties

Etude des proprietes magnetiques a basse temperature des phases de Chevrel. Interaction entre supraconductivite et magnetisme

Structure and magnetic properties of two niobium chlorides with ¦Nb6Cl12¦n+ (n = 2, 3) units: KLuNb6Cl18 and LuNb6Cl18

Abstract The quaternary chloride KLuNb6Cl18 and the ternary chloride LuNb6Cl18 were synthesized. They crystallize in the rhombohedral lattice R 3 and exhibit ¦Nb6Cl12¦2+ and ¦Nb6Cl12¦3+ units respectively. In the first case, the ¦Nb6Cl12¦2+ unit has 16 e− per Nb6 and the cluster is non-magnetic; in the second case, the ¦Nb6Cl12¦3+ unit has 15 e− per Nb6 and the cluster is paramagnetic owing to one unpaired electron. We present here the synthesis of these compounds, the structural results and the magnetic properties which depend on the variation in the oxidation state of the cluster.

Molecular structure and magnetic properties of [Gd(LH4)(NO3)2(H2O)]NO3(H2O)2, [Sm(LH4)(NO3)2(H2O)]NO3 (H2O)1.5(CH3OH)0.5 and [Cu2(LH2)(H2O)2](NO3)2 complexes (LH4: Schiff base ligand derived from 4-methyl-2,6-diformylphenol and 1,3-diaminopropanol)

Abstract Novel lanthanide(III) complexes (Ln=Gd (1), Sm (2), La (3)) of the macrocyclic ligand LH4 have been prepared and characterized. The binuclear copper(II) complex (Cu2(LH2)(H2O)2)(NO3)2 (4), has been obtained by transmetalation of complex 3 with Cu(CF3COO)2. The macrocyclic Schiff base ligand (LH4) is formed by the condensation of two molecules of 4-methyl-2,6-diformylphenol with two molecules of 1,3-diaminopropanol in the presence of the lanthanide ions. The alcohol group of the macrocycle does not coordinate to the copper(II) centers of the binuclear complex. However, one of the secondary alcohol groups of the ligand coordinates in an asymmetric mode to the lanthanide ion in the corresponding three complexes. The X-ray crystal structures of (Gd(LH4)(NO3)2(H2O))(NO3)(H2O)2 (1), (Sm(LH4)(NO3)2(H2O))(NO3)(H2O)1.5(CH3OH)0.5 (2), and (Cu2(LH2)(H2O)2)(NO3)2 (4) have been determined. Magnetic susceptibility measurements in the 5–300 K range were obtained for these complexes.

Crystal structure, electrical conductivity and Seebeck coefficient of Y(Mn, Ni)O3 solid solution

Abstract Solid solutions belonging to the Mn-rich region of the YNixMn1−xO3 system have been studied. The powders were prepared by solid state reaction between the corresponding oxides. Sintered ceramics were obtained by firing at 1325–1350°C. The incorporation of 20 at.% Ni2+ to the yttrium manganite induces the formation of a perovskite-type phase, with orthorhombic symmetry and space group Pbmn. Increase of the Ni amount leads to an increase of the orthorhombicity factor b/a of the perovskite, up to an amount of 50 at.% Ni2+. Above this Ni amount, a biphasic system has been observed, with the presence of unreacted Y2O3. DC electrical conductivity measurement has shown semiconducting behaviour for this solid solution with perovskite-type structure. The room temperature conductivity increases with Ni up to ∼33 at.% Ni, and then decreases. Small polaron hopping mechanism controls the conductivity in these ceramics. Results are discussed as a function of the Mn3+/Mn4+ ratio for each composition. NTC thermistor capability and potential application as ceramic electrode for SOFCs are presented.

Fluorination of YBa2Cu3O6.7 : Powder neutron diffraction determination of fluorine sites and their influence on the superconducting properties

Abstract The crystal structures of YBa2Cu3O6.7 before and after 30 minutes or 75 minutes of fluorination under diluted NF3 at 300°C, were determined by powder neutron diffraction experiments. For short time of fluorination, the occupancy of the (0, 1 2 8:48 AM 6/6/02, 0) site increases while the one of the ( 1 2 , 0, 0) position does not vary too much. For higher fluorine concentration, the (0, 1 2 , 0) occupancy seems to reach a limit, while the ( 1 2 , 0, 0) one further increases. These structural results are discussed in relation with the two different evolutions of the superconducting behaviour previously observed : the enhancement of Tc from 65 K upto 90 K is related to the occupancy of the 0 (4) sites in the CuO chains along the b axis, while filling the 0(5) sites along the a axis mainly affects the a.c. susceptibility amplitude which decreases suddenly from 100% down to about 30%.

Actual fluorination of Y-Ba-Cu-O high-Tc superconductors by a solid-gas reaction under NF3 flow

Sintered samples of YBa2Cu3Ox with x approximately=6, 6.7 and 7 have been reacted with diluted NF3 gas at various temperatures around 300 degrees C. Evidence for fluorine insertion in the material is given by weight increase and by concomitant evolution of the X-ray patterns and superconducting behaviour. The kinetics of the fluorine insertion clearly gives evidence for a direct correlation between the amount of inserted fluorine and the oxygen stoichiometry in the starting samples, namely the vacancy concentration. The fluorination effect on the superconducting properties has been studied by AC and DC susceptibility; after fluorination, for the compound in which x approximately=7, no modification of the Tc onset is observed, but the Meissner effect amplitude decreases in relation to the granularity effect. For the compound in which x approximately=6.7, fluorination increases the Tc onset up to 91 K. Further annealings of the fluorinated samples have been performed under O2, N2 and dynamic vacuum, and their influence on the superconducting behaviour is discussed.

Magnetism and structure in chains of copper dinuclear paddlewheel units.

An anhydrous copper carboxylate compound of formula [Cu(trans-2-butenoate)(2)](n) has been characterized. X-ray analysis reveals a structure built by paddlewheel units bridged by pairs of Cu...O axial bonds to give infinite chains arranged in a new topological motif. Susceptibility measurements in the 10-300 K temperature range, and isothermal magnetization curves at 2, 5, 10, and 50 K with fields up to 5 T, were obtained. Electron Paramagnetic Resonance (EPR) spectra of powder samples were measured at 33.9 GHz at 300 K, and at 9.60 GHz at temperatures in the range 90 <or= T <or= 350 K. Fitting the susceptibility data to a chain model of alternating paddlewheel and [Cu(mu-O)(2)Cu] dinuclear units yielded exchange interactions J(1) = -330.6(1) cm(-1) and J(2) = 5.9(2) cm(-1). The EPR spectra reflect the usual antiferromagnetic dinuclear behavior, with zero field splitting parameters of the excited spin triplet, D = -0.329(3) cm(-1) and E approximately 0, plus a central peak not expected for isolated dinuclear units. We interpret this peak as arising from the stochastic spin dynamics of the chain introduced by exchange couplings between spins in neighbor dinuclear units, which averages out the zero field splitting. Interactions of the units with the rest of the chain acting as a spin bath give rise to a quantum transition between localized dinuclear states and states of the spin chain. This effect competes with the condensation of the antiferromagnetic dinuclear units into the singlet ground state, producing a characteristic temperature dependence of the shape of the powder EPR spectra. We interpret these features in terms of basic theories of magnetic resonance in coupled spin systems applied to the chain array of dinuclear units.