J. Fontcuberta

Crystal growth and phase diagrams for the Nd2O3-CeO2-CuO system

Abstract The phase diagrams for the binary and ternary Nd 2 O 3 -CuO and (0.85Nd 2 O 3 -0.3CeO 2 )- CuO systems are reported and the eutectic points and the liquid lines are determined. We have also explored the crystal growth from molten solutions of compositions 70% CuO-(30- z )% Nd 2 O 3 - z % CeO 2 for a range of CeO 2 concentrations. We show that the composition of the crystals is not uniform for a given z value but the compositional range appears to be quite narrow ( x =0.17(3)) for z =3.6. This concentration produces also the largest crystals (≈10 X 10 X 0.3 mm 3 ).

Structural and magnetic characterization of the lithiated iron oxide LixFe3O4

The Rietveld profile‐analysis method is used to investigate the x‐ray diffraction pattern of lithiated Fe3O4. It is shown that, after exposure to air, pure magnetite coexists with a lithium‐inserted LixFe3O4 phase. The Mossbauer spectra at 300 and 4.2 K have been used to estimate the lithium content of the sample, the pure magnetite concentration, and the iron distribution over the available 16c and 16d sites of the spinel structure. Magnetization measurements from 4.2 to 120 K with an external magnetic field up to 150 kOe have been used to obtain the saturation magnetic moment, the magnetic anisotropy constants, and the susceptibility. It is concluded that a noncollinear spin structure should be present in Li0.5Fe3O4. These results indicate that there is no room‐temperature extrusion of iron even for x→2.0, but that on exposure to air LixFe3O4 samples with x>0.5 are oxidized at room temperature by delithiation.

Mechanism of habit change of ADP crystals by Fe3+, based on Mössbauer studies

Abstract The habit change of ADP crystals grown in presence of iron impurities is studied using Mossbauer spectroscopy. A single structural position for Fe 3+ incorporation has been found. Based on this result, habit change mechanisms proposed in the literature are analysed.

Cationic distribution, magnetization and magnetic anisotropy of Co2+ doped M-type barium ferrite

Abstract The cation distribution and the high field magnetization of the Co-Ti M-type barium ferrites have been investigated by means of neutron powder diffraction and high field isothermal magnetic measurements. The composition dependence of the experimental magnetization is compared to that obtained from the cationic distribution assuming a collinear magnetic structure. A considerable amount of Co ions occupy tetrahedral sites being ineffective in the reduction of the uniaxial magnetic anisotropy.

High temperature superconductor composites by a modified bridgman method

Abstract We have investigated single-crystal growth of high temperature superconductor (111z) Bi-Sr-Ca-Cu materials by using a directional solidification method. We show that for a certain range of z, quaternary crystals having an essentially homogenous atomic composition can be obtained. For larger z values, dense composites are formed by alternate layers along the c-axis of the superconducting quaternary oxide and a semiconducting ternary Sr-Ca-Cu oxide. Preliminary transport measurements of these sandwiches reveal a superconducting behaviour within the ab plane but semiconductor-like along the c-axis. Plate-like single crystals of the HTSC Bi2.2Sr2Ca0.7Cu2Oy with dimensions up to 5 × 7 mm2 × (50–100) μm have been prepared by this method.

Dynamics of the bipyramidal ions in SrFe12O19 studied by Mossbauer spectroscopy

The dynamics of the bipyramidal Fe sites in SrFe12O19 is investigated. Using single crystal Mossbauer spectroscopy data, the authors have shown that these Fe ions diffuse rapidly between two pseudo-tetrahedral sites (4e) in a wine-bottle-shaped potential at any temperature (70-300 K). The effective recoilless fraction associated with the 4e sites is described in terms of a self-interference mode, which provides a measure of the jumping distance. It is found that the jumping distance strongly decreases when the temperature is decreased. The room-temperature data are in excellent agreement with the available X-ray diffraction data.

Ferromagnetic interactions above room temperature in a Schiff-base metal-organic polymer

Abstract The magnetic properties of an iron-bonded Schiff base polymer of general formula {[Fe(C13H17N3)2]SO4.6H2O}n are reported. Mossbauer spectroscopy shows that the polymer contains about 20±8 % of Fe3+ ions, despite the fact that only Fe2+ is used in the synthesis. A large out-of-phase component of the a.c. magnetic susceptibility is observed at zero external magnetic field and temperatures as high as 350 K. This result is characteristic of the presence of net magnetic moments in the material. The saturation magnetization is estimated to correspond to a saturation magnetic moment of 0.10 μB per structural repeating unit of the polymer.

Influence of internal electric field on the transport properties of the magnetoplumbite system BaFe12-xMnxO19

Measurement of the room temperature conductivity and Seebeck coefficient for the magnetoplumbite system BaFe12-xMnxO19, 0 Tc, where Tc is the Curie temperature. E1 approximately=0.37 eV appears to be a motional enthalpy which is essentially independent of x, but E2>E1 at x=0 changes to E2

Quantitative analysis of a Fe3O4+LixFe3O4 sample by the X-ray Rietveld method

The Lix Fe3O4 phase shows a strong peak-overlapping with those of pure magnetite, and a routine X-ray analysis does not allow the detection of the presence of any Fe3O4. However, a careful inspection of the observed calculated difference pattern has led to the deduction of the presence of Fe3O4. This paper shows that the Rietveld method can be used to obtain significant information about the relative concentration of the components of a two-phase sample, even in the case of poor crystallinity, strong overlapping between the diffraction peaks, and only a partially known structure.

The dynamics of bipyramidal ions in magnetoplumbite-like hexagonal ferrite systems revisited

The dynamics of the bipyramidal 4e−Fe ions in the M-type hexagonal ferrite La0.7Na0.3Fe12O19 single crystal, is investigated by using Mössbauer spectroscopy. We show that a photon self-interference phenomenon can be used to explain the available experimental data. From this model we have deduced the jumping distance of the Fe(4e) ions. This model can also be succesfully used to investigate the existing data on the isomorphous CaAl11FeO19 and BaFe12O19 oxides. In all cases, the calculated jumping distances are in a excellent agreement with X-ray diffraction results. For CaAl11FeO19 and BaFe12O19 we have deduced the thermal dependence of the jumping distance. Any evidence of an atomic freezing is not found in the temperature range 80–300 K.