Influence Of Oxygen Stoichiometry Research Articles

Magnetic and transport properties of the room-temperature ferrimagnetic semiconductor Fe1.5Ti0.5O3±δ: Influence of oxygen stoichiometry

Fe 1.5 Ti 0.5 O 3 ± δ epitaxial thin films have been grown on α-Al2O3 (0001) substrates by pulsed laser deposition technique. The samples are both ferrimagnetic and semiconducting beyond room temperature. Oxygen pressure (PO2) during the deposition appears to be a critical parameter in promoting high temperature long range magnetic order and semiconducting properties. For all oxygen pressures, Fe1.5Ti0.5O3±δ thin films are single phase with twin epitaxy. High dc conductivity and low magnetization are obtained at low PO2, whereas high saturation magnetization and low conductivity stand for high PO2. Oxygen vacancies and∕or change of iron valence state are pointed out to be responsible of these properties. Superexchange mechanism via oxygen bonds seems rather to dominate the magnetic properties especially for high PO2, whereas for low PO2 a double exchange mechanism might occur. Fe1.5Ti0.5O3±δ appears thus to be an interesting material for high temperature spintronics applications.

Influence of oxygen stoichiometry on the spin-glass behavior ofLiCrMnO4−δspinels

Magnetic properties of the geometrically frustrated spinel antiferromagnets ${\mathrm{LiCrMnO}}_{4\ensuremath{-}\ensuremath{\delta}}$ $(0<~\ensuremath{\delta}<~0.081)$ have been studied with dc and ac magnetic susceptibility and heat capacity measurements, which all give evidence of spin-glass behavior. These compounds exhibit a spin-glass transition ranging from 13 K for $\ensuremath{\delta}=0$ to 15 K for $\ensuremath{\delta}=0.081.$ With increasing $\ensuremath{\delta},$ peak of the magnetic heat capacity is progressively rounded and the magnetic entropy is lowered. This suggests that the number of spins participating in the spin-glass state is rather sensitive to the level of lattice defect.

Influence of oxygen stoichiometry on the electronic properties of La4Ni3O10±δ

Some different oxygen content La4Ni3O10+δ (δ=0.02,0.12,−0.22) compounds were prepared in order to study the influence of Ni3+ content on the electronic properties. A metal–metal transition was detected at 140 K for the as-prepared (La4Ni3O10.02) and reduced (La4Ni3O9.78) compounds. From the electrical conductivity, Seebeck coefficient and magnetic studies of the different samples, oxygen stoichiometry was found to be a critical parameter in defining the existence of a less conductive low temperature state. The results are interpreted assuming the existence of charge order in the nickel oxide planes of the layered perovskite-type structure.

Influence of oxygen stoichiometry on electrical transport and magnetic properties of doped perovskite-type ferrate and manganate single crystals

The electrical transport and magnetic properties of (La 1− x Sr x )FeO 3−δ and (La 1− x Sr x )MnO 3−δ single crystals were studied as a function of the dopant concentration x and the oxygen deficiency δ. Crystals were grown by the floating zone process and subjected to post-annealing treatments in oxidizing or reducing atmosphere to control δ. This two-step process allows the precise control of δ. Moreover, a wide range of δ values not attainable by changing only the growth conditions can thus be studied. Controlling the Fe and Mn valencies via the oxygen deficiency was found to affect the magnetic and electrical properties profoundly.

Crystal growth and the influence of oxygen stoichiometry on electrical resistivity of single crystals

This paper presents the results of an analysis of the effects of reduction and oxidation processes on single crystals of (NCCO). It is experimentally shown that when the oxidation process is cyclically repeated the associated reduction process increases the of NCCO single crystals. The influence of the gassy atmosphere and the rate of the temperature change during thermal treatment on the electrical resistivity of single crystals is widely discussed.

X-ray single crystal analysis of HgBa 2Ca 2Cu 3O 8+δ and influence of oxygen stoichiometry on the superconducting properties

X-ray diffraction analyses of good-quality single crystals of HgBa 2Ca 2Cu 3O 8+δ, synthetized by a closed-vessel technique at low pressure, have lead to more definite results with respect to previous structural studies. Samples with T c = 135 K showed a Cu occupancy of 16.1% on the Hg site at the origin of the unit cell. Excess oxygen appears to be present only in the basal plane at the interstitial site 1 2 , 1 2 , 0 with an occupancy δ = 0.190 ± 0.015. This compound has the shortest copper oxygen apical distance within the mercury family (2.696(3) Å) together with an almost complete planarity of the CuO 2 planes. Variations in the excess oxygen content upon different heat treatments were analyzed by thermogravimetry and magnetic-susceptibility measurements. A correlation between δ and T c could be derived, showing in particular that superconductivity exists down to a very low interstitial oxygen content.

High-Tcsuperconductivity: New applications of ferroelectrics at microwave frequencies

Abstract The initial success in high-T c superconductivity applications at microwaves leads to the creation of new branch in microwave electronics operating at liquid nitrogen temperature. In this connection one should reconsider the behaviour of nonlinear dielectrics at microwaves. The characteristics of a capacitor in the form of trilayer structure YBa2Cu9O7−x/(BaxSr1−x)TiO9/YBa2Cu9O7−x are discussed. The crucial influence of oxygen stoichiometry on the structure properties is assumed. The layered structures including non-linear dielectrics could be used as a basis for devices with voltage control of the microwave circuit parameters.

Influence of oxygen stoichiometry on the antiferromagnetic ordering of single crystals of La2NiO4+ delta.

We report here on magnetic measurements of well-characterized single crystals of ${\mathrm{La}}_{2}$${\mathrm{NiO}}_{4+\mathrm{\ensuremath{\delta}}}$, for H both parallel and perpendicular to the c axis, and for compositions in the range 00.067. The antiferromagnetic ordering temperature is shown to be very sensitive to the oxygen stoichiometry parameter \ensuremath{\delta}. A discontinuity in the \ensuremath{\delta} dependence of the N\'eel temperature has been observed and is attributed to a change in the structure. For 0.055\ensuremath{\le}\ensuremath{\delta}\ensuremath{\le}0.067 we report a time dependence of the remanent magnetization in the basal plane, which suggests that an anisotropic spin-glass phase exists over a narrow composition range in this system.

Influence of oxygen stoichiometry on Tc and pinning force of Bi2Sr2CaCu2O8+δ

Abstract The influence of oxygen stoichiometry in T c and F p of Bi 2 Sr 2 CaCu 2 O 8+δ has been investigated by means of X-ray and electron diffraction, a.c. susceptibility and d.c. magnetization. Through proper oxygen control, T c = 94 K and an increase of one order of magnitude in F p with respect to the started material has been achieved.

Influence of oxygen stoichiometry on the Hall effect of single crystals of YBa 2Cu 3O y

We report Hall effect measurements on YBa 2Cu 3O y single crystals for 6.5⩽ y⩽7. Just below T c superconducting fluctuations lead to non-linear variations of the Hall voltage against the magnetic field. In the normal state the Hall coefficient strongly increases with oxygen removal and displays at higher temperatures a T −1 variation.

Inhomogeneity and Microstructure in e-Beam Evaporated ZrO2 Films

ABSTRACTThin films of zirconium dioxide are deposited by e-beam evaporation on optically polished borosilicate crown glass. Two different oxygen partial pressures in the chamber are used. The optical properties of the films are characterized by ellipsometry. The influence of oxygen stoichiometry on the composition and microstructure of the material is investigated by polycrystalline X-ray diffraction for different film thicknesses. The films are found to be inhomogeneous, and a composition gradient (i.e. amorphous ⇔ tetragonal ⇔ monoclinic) is observed from the substrate to the surface. The oxygen partial pressure influences the growth of the films.

Lattice vibrations in high-T c superconductors: Optical spectroscopy and lattice dynamics

Raman and IR-measurements on the high- T c material series La 2CuO 4 and YBa 2Cu 3O 7 are reviewed. This summary focuses on lattice vibrations in these superconducting systems, on impurity phases, the influence of oxygen stoichiometry and element substitution. The determination of the superconducting gap, the phonon renormalization in the superconducting state, and the isotope effect with its implications on the theory of superconductivity in these materials are discussed. In addition, the measured lattice vibrations are compared with the results of lattice dynamics calculations.

Upper critical field measurements in high-T c superconducting oxides

Using a pulsed quasi-static magnetic field we measured the magnetoresistance of high- T c superconducting oxides and we deduced the values of the upper critical field near T c. The influence of oxygen stoichiometry is discussed.

Influence of oxygen stoichiometry on the structure and superconducting transition temperature of YBa 2Cu 3O x

A detailed study of the superconducting properties and the crystal symmetry of YBa 2Cu 3O x as a function of oxygen content (x) is presented. We correlate the oxygen content, structure and superconducting transition temperature for YBa 2Cu 3O x (6<×<7) series of materials formed by the topotactic intercalation/deintercalation of oxygen. It is shown that the orthorhombic to tetragonal phase transition coincides with a loss in superconductivity for samples prepared both by quenching from high temperature and samples prepared by deoxygenation at low temperature. For the orthorhombic phase, T c monotonically decreases as x goes from 7.0 to 6.4 along with a complementary decrease in the extent of orthorhombic distortion. The decrease in T c, however, is not uniform. For quenched samples it shows a plateau for x ∼ 6.75 to 6.55 and then a rather abrupt drop around x ∼ 6.5. Comparison of our data with the literature indicates that the dependence of superconducting properties and crystal structure on the oxygen content can be a complex function of sample processing history. Samples with the same oxygen content but prepared in different ways may have x-ray powder patterns that are indistinguishable, but significantly different electrical properties.

The influence of oxygen-stoichiometry on desulfurization during FBC: A simple sure modeling approach

In this paper an extension is presented cf the simple SUlfur REtention SURE model which was published earlier by the authors (Schouten and Van den Bleek, 1987). The present model explains the effect of a decreasing desulfurization efficiency during staged Fluidized Bed Combustion of coal by the relatively fast formation of SO 3 as a gaseous intermediate reactant in the sorbent sulfation reaction. The SURE model provides an analytical expression for the molar (Ca/S) ratio in the reactor feed as a function of the degree of sulfur retention, the maximum sorbent conversion and three dimensionless model parameters. These parameters are functions of fluid bed operating conditions, coal and sorbent properties and the stoichiometric air ratio. It is shown that the oxygen concentration and the decrease in sulfur retention at lower stoichiometric air ratios can readily be described by the model equations. Furthermore, the SURE model is applied as a diagnostic tool for the analysis of plant operational data obtained from the recent literature. It is concluded from an evaluation of SURE model calculations and experimental data that the sorbent residence time is a crucial system parameter. It is demonstrated that the observed neglect of the influence of the sorbent residence time on the steady state character of many retention experimentations gives rise to serious doubt on the reliability and usefulness of these measurements for model verification purposes.