Synthesis and Physical Properties of (Cu,M)Ba2Ca3Cu4Oz (M=C,Mg,Ni,Al,Zn,Tl)

We have synthesized the single phase samples of Cu _x C _0.1 Ba ₂ Ca ₃ Cu ₄ O _z (x=0.20.9). Transport and magnetic measurements and iodometric titration analysis were carried out for these materials. From these results we discuss the correlation between T _c , the Hall number, copper valence and oxygen content. The effect of the substituted M ions on T _c in (CuC, M)-l234 (M=Ni,Zn,Al,TI,Mg) is also discussed. We propose a unique selective carrier-doping mechanism in (Cu,C)- and (CuC,Mg)-1234 systems.     

Role of oxygen content in the structural and the superconducting properties of high temperature superconducting ErBa2Cu3O y

Samples of high temperature superconducting system ErBa₂Cu₃O _y prepared by the standard ceramic technique were found to have single phase. To vary the oxygen content in the samples, the mother compound was reduced by vacuum-annealing at different temperatures for a fixed period. The oxygen content was measured by gravimetric and iodometric titration procedures. The structural changes induced due to the variations in the oxygen content were studied by X-ray methods.     

On the effects of Ti,Zr and Ce substitution in YBa2Cu3O7−δ : transport,XRD, XPS and XANES studies

Results of the XRD, XPS, XANES, iodometric titration and transport studies on freshly prepared Ce, Zr and Ti substituted 1-2-3 samples are reported here. It is argued that at least 2% of Ce, Zr and Ti ions go into the 1-2-3 lattice. It is found that Zr, Ti ions exist in 4⁺ valence state, Ce is in predominantly 4⁺ (mixed valence) state and valence of Cu remains unaffected on substitution.     

Determination of the bulk cobalt valence state of co-perovskites containing surface-adsorbed impurities

We used thermogravimetric hydrogen reduction and iodometric titration to determine the bulk valence state of cobalt in Co-perovskites containing surface carbonate hydroxide or hydroxyl groups. It could be shown that thermogravimetric hydrogen reduction experiments are very sensitive to volatile surface groups, but due to their volatility, they can be specified and the bulk valence state of cobalt can still be deduced from these experiments. The iodometric titration is less sensitive to small volatile surface impurities, but precaution has to be taken that oxygen or iodine does not escape from the solution during dissolution of the sample. Best results were obtained if the sample was titrated during dissolution in a closed argon floated titration apparatus. We tested the two methods using LaCoO3 perovskite as a sample with a known valence state. Both methods delivered satisfactory results, and the valence state could be determined with an accuracy of better than 1%.     

Oxygen content determination in perovskite-type cobaltates

Three oxygen content determination methods (iodometric titration, gas volumetric analysis and thermogravimetric hydrogen reduction) have been evaluated for the case of rare earth cobaltates LnBCo₂O_5+δ (were Ln-rare earth, B-alkali earth elements). All the methods are based on oxidation properties of both Co⁴⁺ and Co³⁺ cations but different instrumentation was used for each of the method. We have obtained good agreement within ±0.02 in δ for both the iodometric titration and the gas volumetric analysis, whereas the thermogravimetric hydrogen reduction was found to be less credible.     

Preparation of superconducting-grade copper from commercial grade copper salt

Fine samples with nominal composition of Bi_1·6Pb_0·4Sr₂Ca₂Cu₃O _x have been produced by solid state method using various purity grades of starting copper oxide powder. Studies onT _c and high-T _c volume fraction measurements of these samples revealed that the samples produced using CuO powders obtained in laboratory after double purification of the commercially available copper salts have higherT _c (104·46 K) and increased percentage of highT _c volume fraction (58%) compared to even the samples prepared from Aldrich grade (99·99%) CuO. A simple and cost-effective chemical route for the purification of CuO from commercially available copper salts has been outlined.     

Oxygen Exchange in Pr1 + x Ba2 – x Cu3O6 + z Substitutional Solid Solutions

Pr_{1 + x} Ba_{2 – x} Cu₃O_{6 + z} substitutional solid solutions prepared by solid-state reactions in air, using the appropriate metal nitrates as starting reagents, were studied by x-ray diffraction, microstructural analysis, and thermogravimetry (TG). The oxygen content of the synthesized materials was determined by iodometric titration. The TG data obtained at different heating rates were used to evaluate the apparent activation energy E _a for oxygen exchange between the solid and gas phases. The results indicate that the E _a for Pr_1.4Ba_1.6Cu₃O_{6 + z} is much higher than those for the materials withx= 0, 0.25, and 0.6. The anomaly in E _a correlates with the variations in some other physicochemical properties of the solid solutions.     

An equation to calculate oxygen content relating to copper oxidation level of doped cuprates

A new equation relating oxygen contenty to copper valence 2+p has been derived and proposed. Compared to the previous equation, there is a corrected function in the new equation. The corrected function depends on dopant contentx and copper oxidation levelp. As a metalM is substituted for the metal of the sample, the corrected function has a different form, and the function is discussed in detail. For each case, taking high-T _c cuprate for example, itsp is determined andy calculated by the new and previous equations. Changes in the corrected function are shown in the figures. All the discussions show that it is necessary to use the new equation to calculatey if we are to get accurate experiment results. The new equation can be widely applied to calculate the oxygen content of multicomponent doped or undoped cuprates, and the exact stoichiometries and superconducting or impurity phases of samples need not be known.     

An improved procedure for determining oxygen content in high-T ccopper oxides

We present an improved double titration method for high-T _c superconductors, in which the major step is adding KSCN as an indicator of the end point after the starch end point. By adding it, a greater amount of Na₂S₂O₃ solution can be added after the starch end point, at least 0.12 ml. We found that adding KSCN can lead to precise measuring results and reduce the experimental error. The oxygen contents of two series of superconductors were determined by this approach. For YBa₂Cu₃O _y , the errors are Δp～0.003 and Δy～0.005, which are lower than that of other reports.     

Structural studies on Fe-substituted Y-Ba-Cu-O superconductors by Mössbauer spectroscopy

In this paper we report preparation and structural aspects of YBa₂Cu_2·97Fe_0·03O_7?x with different oxygen stoichiometry. The samples were prepared by standard solid-state reaction. The samples were characterized by X-ray powder diffraction and oxygen contents were estimated from iodometric titration method. The superconducting transition temperatureT _c is measured using the standard four-probe technique. Mössbauer studies of YBa₂Cu_2·97Fe_0·03O_7?x with different oxygen stoichiometry were carried out and the results show that Fe substitutes both Cu(2) and Cu(1) sites and clear asymmetry in the doublets except for the tetragonal case. Also our results show the progressive oxygenation inb-axis as oxygen stoichiometry is increased.     

An improved procedure for determining oxygen content in high-Tccopper oxides

We present an improved double titration method for high-T _c superconductors, in which the major step is adding KSCN as an indicator of the end point after the starch end point. By adding it, a greater amount of Na₂S₂O₃ solution can be added after the starch end point, at least 0.12 ml. We found that adding KSCN can lead to precise measuring results and reduce the experimental error. The oxygen contents of two series of superconductors were determined by this approach. For YBa₂Cu₃O _y , the errors are p0.003 and y0.005, which are lower than that of other reports.     

About copper valence and superconductivity

We describe the copper valence in superconductors based on our arguments on La₂O₃ crystal structure. In order to explain the two oxygen sites in La₂O₃, it has been supposed that O _II ¹⁻ ions occupy the tetrahedral site while O _II ²⁻ occupy the octahedral site. Oxygen ions in tetrahedral site form a covalent bond with lanthanum, which can be written [LaO]¹⁺. Considering the chemical and crystallographic properties of Tl and Bi compounds, [TlO]¹⁺ and [BiO]¹⁺ groups appear as defined by strong covalent bonds between Bi or Tl and O. This leads to the supposition that the four oxygen ions which coordinate to copper in Tl and Bi copper oxide-based superconductors are O _II ¹⁻ ions. The bivalent character of copper is then obtained through covalent bonds. For La_2−x Sr _x O₄ compounds, copper is supposed to have valence three, but spectroscopic studies point out bivalent copper. We show that krypton shell of Sr is responsible for the lack of one unit of valence as expected from krypton compounds for example KrF₂.     

Synthesis and transport properties study of Nd1−xSmxNiO3−δ solid solutions

Nd_1−xSm_xNiO_3−δ ceramics were studied by means of XRD, iodometric titration, DSC and resistivity measurements. The metal-insulator transition for all compositions was studied by two independent procedures: resistivity and DSC measurements. The reason for differences in transition temperatures, defined by these two techniques is discussed. It was shown that these materials are promising for practical applications in bolometric devices due to high values of temperature coefficient of resistivity at room temperature.     

The effect of replacing copper metal or oxide with neodymium on the optical properties of lithium tetraborate glass

The melting quenching technique is used to create lithium tetraborate glasses with the following composition (in mol %): 80 Li₂B₄O₇-(20-x) Nd₂O₃-x Cu metal or x CuO mol% (x?=?0, 5, 10, 15, and 20). X-ray diffraction (XRD), optical absorption, photoluminescence, thermal analysis, and Fourier transmission infrared (FTIR) were studied. The optical absorption spectra indicate the presence of Nd³⁺ and Cu²⁺ ions. The absorption spectra determine the Judd–Ofelt intensity parameters (Ω_λ), the oscillator strengths, and bonding parameters. The high value of Ω₂ indicates the increasing covalent bonding within the host structure. The ratio between emission intensities and excitation indicates that glass samples are used as laser sources. The photoluminescence spectra revealed neodymium ion emission. The glasses containing 20 mol % Nd₂O₃ (free from copper) emitted light at 1053 nm. The strength of emission peaks decreases as copper concentration increases according to the quenching effect. The findings indicate that the samples investigated are suitable for use in solid-state lasers.

     

Unusual magnetic and transport properties of the CMR Pr<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ca<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3−<Emphasis Type="Italic">y</Emphasis></Subscript> system

The substituted nonstoichiometric perovskite Pr_1−x Ca _x MnO_3−y compounds have been synthesized by a standard combustion technique, which show uniphase solid solutions. The all samples of the Pr_1−x Ca _x MnO_3−y system show an orthorhombic crystal system and the cell volumes are decreased with increasing the larger amounts of substituted atoms or the increasing x values. The mixed valence of Mn ions is identified by the XAS (XANES/EXAFS) spectroscopy and the amounts of Mn⁴⁺ ions are determined by an iodometric titration method. Nonstoichiometric chemical formulas of the Pr_1−x Ca _x Mn_1−τ³⁺Mn_τ⁴⁺O_3−y compounds have been obviously formulated. Magnetic properties are investigated by SQUID and thus the Pr_1−x Ca _x MnO_3−y (x = 0.4, 0.6, and 0.8) compounds show the transition from antiferromagnetic state to paramagnetic state. The Pr_1−x Ca _x MnO_3−y (x = 0.0, 0.2, and 1.0) compounds show the transition from ferromagnetic state to paramagnetic state. The facts that Mn⁴⁺ contents play important roles in the magnetic ordering have been found out. The transport properties have been studied by the DC electrical conductivity measurement under magnetic fields of 0 G and 3 kG. Maximum and minimum MR ratios are 1016% of the Pr_0.6Ca_0.4MnO_2.846, and −77.5% of the PrMnO_3.021 compound, respectively.     

Nature of chemical bonding in ThF<Subscript>4</Subscript>

The fine structure of the X-ray photoelectron spectrum of ThF₄ in the range of valence electrons (binding energy from 0 to 35 eV) is examined. The analysis takes into account the results of theoretical calculation of the electronic structure of the ThF₈⁴⁻ cluster (point group C ₂) as a model of the nearest surrounding of the Th atom in ThF₄, performed by the relativistic discrete variation method. It is demonstrated theoretically and confirmed experimentally that formation of a chemical bond gives rise to filled states of Th5f electrons (∼0.5 Th5f electron) in the energy range of electrons of outer valence molecular orbitals (valence band). The Th6p electrons noticeably participate in formation not only of inner valence but also of outer valence (∼0.4 Th6p electron) molecular orbitals. The composition and order of inner valence molecular orbitals in the energy range from 13 to 35 eV is determined, and the density of states of valence electrons in the range from 0 to 35 eV in ThF₄ is calculated. The results obtained allowed the fine structure of the high-resolution O _4,5(Th)-emission spectrum of thorium in ThF₄ in the photon energy range from ∼60 to ∼85 eV, associated with the formation of outer and inner valence molecular orbitals, to be interpreted for the first time.     

Influence of the nature and density of defects on the thermodynamic and electrical properties of semiconductor materials

This paper examines the use of coulometric titration in studies of the nonstoichiometry, defect structure, and thermodynamic properties of semiconductor materials and also for doping of a number of chalcogenides with lead, copper, and germanium. A direct relationship is established between the thermodynamic and electrical properties of the binary compound semiconductors Pb_{1 ± δ}X (X = S, Se, Te), Cu_{2 ± δ}Se, and Cd_{2 ± δ}Se and the ternary spinel semiconductors Cd_{1 ? δ}Cr₂Se₄ and Cu_{1 ± δ}Cr₂S₄ within their homogeneity ranges. The width and symmetry of the homogeneity range of these semiconductor materials are determined using coulometric titration in combination with emf and electrical conductivity measurements. Electrochemically doped nonstoichiometric copper selenide samples are shown to have compositions in the range Cu_1.27Se–Cu_2.73Se.     

Electron spin resonance in copper phosphate and copper tellurite glasses containing strontium

Two series of ternary copper phosphate and copper tellurite glasses containing strontium were prepared and the electron spin resonance spectra of glasses of compositions 65 (P₂O₅)-(35-x) CuO-xSrO and 65 (TeO₂)-(35-x) CuO-xSrO where x varied from 0–10 mol % were investigated. From the results and the chemical analyses of the samples it was found that a reduction in the copper (II) signal intensity in the glass samples as the proportion of alkaline earth metal is raised, corresponds to an increase in the reduced valency ratio, C, in the glasses.     

Anomalous Pr Ordering, Pr L 3-Edge and Cu K-Edge XANES Studies for the Insulating PrBa2Cu3O7−y System

X-ray absorption near-edge structure (XANES) data for the PrBa₂Cu₃O_7–y (6.217–y6.95) insulating system with anomalous Pr antiferromagnetic order are reported. Pr L3-edge XANES indicates that Pr formal valence decreases from 3.17–3.18 for oxygenated samples to 3.09 for oxygen-depleted PrBa₂Cu₃O_6.43, 3.03 for PrBa₂Cu₃O_6.38 and 3. 01 for PrBa₂Cu₃O_6.21. The variation of Pr valence indicates that the doped holes initially stay in the CuO_1–y plane and contribute to Cu(I)–Cu(II) transformation in this plane, but gradually move into the CuO₂ bi-layers with PrO₈ cage and result in the appearance of Pr(IV) localized-hole state with, increasing oxygen content. Cu K-edge XANES indicates that the average Cu formal valence increases from less than 2 in oxygen-depleted region to a value larger than 2 for oxygenated samples. with the estimated Cu valence of 2.24 for PrBa₂Cu₃O_6.95. The small average Cu formal valence or large Pr formal valence is closely related to the absence of superconductivity.     

Effect of zinc and iron on the (micro-)structure and copper charge excess of the YBaCuO superconductor

Samples of YBa₂(Cu_1–y Zn _y )₃O_7–y and YBa₂(Cu_1–y Fe _y )₃O_7+x , withy in the range 0.0–0.16 for zinc and 0.0–0.30 for iron were synthesized by solid-state reaction. The solubility limit has been found to be equal to 7% and 19 at % for zinc and iron, respectively. Zinc has little effect on the structure which remains orthorhombic throughout all the zinc concentration range, while iron induces an important structural modification (tweed structure formation). Both impurities induce grain-growth inhibition but densification appears to be independent of the impurity content with respect to the undoped material. However, iodometry shows that the oxygen stoichiometry decreases in zinc-doped samples while it increases in iron-doped samples. Charge balance resulting from the dopant charge and the evolution of the copper charge with doping have been invoked. In zinc-doped samples, the copper charge excess (copper charge fraction>2) decreases sharply for 0.0y<0.04, then it shows a plateau-like behaviour for 0.04y<0.06, while in iron-doped samples, it decreases almost monotonically. Confirming some of our previous results there is a correlatedT _c decrease in the case of zinc-doped samples (occurrence of aT _c plateau) and in the case of iron-doped samples (quasi monotonical decrease). This difference has been interpreted in terms of structural changes related to the different substitution behaviour of zinc and iron.