Investigation of the dependency of the upper critical magnetic field on the content x in Y1−xYbx/2Gdx/2Ba2Cu3O7−y superconducting structures

The Y_1−xYb_x/2Gd_x/2Ba₂Cu₃O_7−y superconducting samples for x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 were prepared by using the solid-state reaction technique. Resistivity measurements of the samples were performed in QD–PPMS system under different magnetic fields up to 5 T in zero fields cooling regime. Using the resistivity data, the upper critical magnetic field H_c2(0) at T = 0 K for 50% of R_n was calculated by the extrapolation H_c2(T) to the temperature T = 0 K. The coherence length in T = 0 K were calculated from H_c2(0) and the effects of x in the composition on both the coherence length and the upper critical magnetic field were examined. The results showed that H_c2(0) varied from 84.05 to 122.26 T with the content x. The upper critical magnetic field in the temperature T = 0 K slightly decreased with increasing the content x. Using the content x, the upper critical magnetic field can be controlled and this can be used in the superconductivity applications.     

Melt processed YBa2Cu3O7−x superconductors

The superconducting YBa₂Cu₃O_7−x samples were prepared by an Arc-Cast-Annealing (ACA) and Arc-Quench-Powder-Growth (AQPG) processes as modifications of QMG and MPMG techniques. Pe'lets of YBa₇Cu₃O_7−x were quenched by arc-casting in a water cooled copper mould and then the solidified rods were annealed at different temperatures and times to store the superconductivity. Annealed at an appropriate temperature the cast rods showed rising superconducting properties with increasing the annealing time. Some of the rods after solidification were crushed to give powder which was compacted and then subjected to a melt growth process. As a result of this processing, large grained textured YBCO superconductors with dispersed 211 inclusions in the superconducting grains were produced. The microstructure and physical properties of these ACA and AQPG samples were investigated when subject to various temperature cycles. It was found that the volume fraction and size distribution of the second phase inclusions were dependent upon the maximum temperature during the melt growth process. The critical current density (J_c) for ACA and AQPG samples was estimated from magnetization loops using Bean's critical state model. It was found that the value of J_c of AQPG sample was much higher than that of ACA sample.     

Magnetic properties of Pr1−xGdxMn2Ge2 compounds

The structure and magnetic properties of the Pr_1−xGd_xMn₂Ge₂ (0.0≤x≤1.0) compounds have been investigated by means of X-ray diffraction (XRD), differential scanning calorimetry (DSC) techniques and AC magnetic susceptibility measurements. All compounds crystallize in the ThCr₂Si₂-type structure with the space group I4/mmm. The lattice constants and the unit cell volume obey Vegard’s law. Samples in this alloy system exhibit a crossover from ferromagnetic ordering for PrMn₂Ge₂ to antiferromagnetic ordering for GdMn₂Ge₂ as a function of Gd concentration x. At low temperatures, the rare earth sublattice also orders and reconfigures the ordering in the Mn sublattice. The results are summarized in the x–T magnetic phase diagram.     

Synthesis and characterization of new pyrochlore solid solution Bi1.5Sb1.5Cu1−xMnxO7

A new pyrochlore solid solution with formula Bi_1.5Sb_1.5Cu_1−xMn_xO₇ has been synthesized using ceramic method at 1000 °C. The cell parameter decreases linearly with increasing manganese concentration. Rietveld refinements for (B_1.5Mn_0.5)(Sb_1.5Mn_0.5)O₇ compound using X-ray powder diffraction data confirmed an overall A₂B₂O₇ cubic pyrochlore structure with a = 10.42749 (4) Å and Fd-3m symmetry. The reliability factors are R_wp = 3.48%; R_p = 2.37%; R_exp = 1.65% and R_Bragg = 1.58%. The magnetic susceptibility measurements achieved between 4 and 300 K indicate a paramagnetic behaviour with an oxidation state “2+” of the manganese ion. The electric resistance measured using complex impedance spectroscopy method put in evidence a decrease of the electric resistance with the temperature, which reached 5 × 10² Ω at 675 K. Dielectric properties depend on the variation of frequency and temperature, results indicate a conductive compound.     

Structure and magnetic properties of Gd3(Fe1−xTix)29 (x=0.011–0.034)

Single-phase compounds Gd₃(Fe_1−xTi_x)₂₉ (x=0.0110.034) have been synthesized. Gd₃(Fe_1−xTi_x)₂₉ crystallises in a monoclinic lattice with space group P2₁/c, and the crystal structure is refined by the Rietveld technique based on X-ray powder diffraction data. Thermomagnetic analysis indicates that the Curie temperature of the compounds ranges from 517 K to 538 K. The saturation magnetizations of the Gd₃(Fe_1−xTi_x)₂₉ (x=0.011, 0.022, 0.034) at 1.5 K are 103.6, 102.0 and 94.3 Am²/kg, and the anisotropy fields at 1.5 K are 6.0, 6.2 and 6.4T, respectively.     

Subsolidus phase relation and crystal structure of the PrBa2−xSrxCu3O7±δ system

The PrBa_2−xSr_xCu₃O_7±δ solid solution was investigated by means of X-ray powder diffraction in combination with Rietveld analysis. The Sr-doped Pr123 single phase could be synthesized at 950 °C in air. The solubility of PrBa_2−xSr_xCu₃O_7±δ solid solution is 0.2≤x≤0.6. The structure of PrBa_2−xSr_xCu₃O_7±δ is orthorhombic for x=0.2. The structure transforms into tetragonal for 0.3≤x≤0.6. In the PrBa_2−xSr_xCu₃O_7±δ structure, Sr ions can replace Ba ions, the highest value is x=0.6 under our experimental condition. But Sr ions could not replace Pr ions. Furthermore Pr ions could not occupy the sites of Ba ions in the PrBa_2−xSr_xCu₃O_7±δ system. Both ionic radii and chemical properties play an important role in the mutual substitution of Pr, Ba and Sr ions in the Pr123 structure of the PrBa_2−xSr_xCu₃O_7±δ system.     

Structural, magnetic and magnetocaloric effect in double perovskite Ba2CrMo1−xWxO6

Magnetic refrigeration, an emerging new technology for cooling and gas liquefaction, needs magnetic materials with specific thermomagnetic behavior. Depending on the thermodynamic cycle selected, the isothermal magnetic entropy change or the adiabatic temperature change upon field application needs to be a preselected function of temperature. In double perovskite Ba₂CrMo_1−xW_xO₆ (x = 0, 0.2 and 0.5) prepared by the sol–gel method, the experimental results show the observation of a large magnetocaloric effect (MCE) near the Curie temperature T_C which decreases with the increasing of the substitution of Mo by W. The maximum of the magnetic entropy change peaks at the magnetic ordering temperature T_C, and a large magnetic entropy change (|ΔS_M| ≈ 1.6 J kg⁻¹ K⁻¹) is obtained at 285 K in the sample Ba₂CrMo_0.5W_0.5O₆ under an applied magnetic field of 10 kOe.     

Magnetic phase transitions in Nd1−xYxMn2Ge2 (0 ≤ x ≤ 0.6)

The structure and magnetic properties of Nd_1−xY_xMn₂Ge₂ (0.0≤x≤0.6) were studied by X-ray powder diffraction and magnetization measurements. All compounds crystallize in the ThCr₂Si₂-type structure with space group I4/mmm. Substitution of Y for Mn led to a linear decrease in the lattice constants and the unit cell volume. Increasing substitution of Y for Nd in NdMn₂Ge₂ shows a depression of ferromagnetic ordering and the gradual development of antiferromagnetic ordering.     

Structural, magnetic and electrical transport properties for a series of La1−xSrxFe1−xMnxO3 (0.3 ≤ x ≤ 0.7) compounds

The structural, electrical transport and magnetic properties have been studied for compounds: La_1−xSr_xFe_1−xMn_xO₃ (0.3 ≤ x ≤ 0.7). The lattice parameter, a, first decreases with x, and followed by an increase when Sr²⁺ and Mn⁴⁺ was continuously doped. The cell parameters, b and c, slightly decrease with coupled substitution of Sr²⁺ for La³⁺ and Mn⁴⁺ for Fe³⁺. In the paramagnetic temperature range, formation of magnetic clusters is suggested; the sizes of clusters decrease with x up to 0.5, following that they increase sharply with continuing doping. The electrical behaviors of all specimens demonstrate insulators and the electrical resistivity increases with content of Mn⁴⁺ and Sr²⁺ ions doped. A variable range hopping model is suitable to describe electrical transport process for the compounds at low temperature. At high temperature the electrical transport process can be described by bipolaron model for all compounds.     

Influence of simultaneous addition of Sb2O3 and SnO2 on thermoelectric properties of Zn1−x−ySbxSnyO prepared by tape casting

Solid aluminum hydride, AlH₃, has been proposed and studied for applications in hydrogen storage. In some samples, a comparatively narrow feature in the proton NMR spectrum is observed; we demonstrate here that this peak is due to molecular hydrogen (H₂) trapped within the solid, presumably from earlier processing procedures or partial decomposition. Static and magic-angle spinning NMR show that the responsible species is highly mobile, even at 11 K. Neutron-energy-gain spectra obtained at 3.5 K yield a feature at or near the free-rotor H₂ energy difference between the J = 1 and J = 0 states. Both NMR and neutron scattering demonstrate ortho–para conversion at low temperatures. Similar NMR signatures in other hydrogen-storage solids such as NaAlH₄ may also be due to trapped H₂.     

Luminescence based on energy transfer in xerogels doped with Ln2−xTbx(WO4)3

This paper presents preparation, optical absorption and photoluminescence properties of luminescent materials consisting of Ln_2−xTb_x(WO₄)₃ [where Ln = Gd(III) or La(III)] incorporated into silica xerogel. Photoluminescence behaviour of the salt in the rigid matrix was studied by the luminescence spectroscopy. The excitation spectra of the system Ln_2−xTb_x(WO₄)₃ show an intense broad band with a maximum placed at about 240 nm. This band is attributed to ligand–metal charge transfer (LMCT) inside the tungstate group. On the other hand, Tb³⁺ ion exhibits its characteristic emission in the material. Owing to energy transfer from the excited tungstate groups to the Tb³⁺ ions the emission intensity is improved. The energy transfer from WO₄²⁻ group to Tb(III) ion is particularly effective for such dopants as Gd_0.4Tb_1.6(WO₄)₃ or La_0.8Tb_1.2(WO₄)₃ incorporated into SiO₂ xerogel. Concentration of the emission quenchers such as water molecules and OH groups was reduced by thermal treatment. The high emission intensity and easy preparation of these systems make them potential candidates for application as luminescent materials.     

Ba4In2−x(CO3)1+xO6−2.5x and Ba4In0.8Cu1.6(CO3)0.6O6.2—new indium oxycarbonates closely related to n=3 Ruddlesden–Popper phases

Investigations of phase relations in the Ba-rich part of the In₂O₃–BaO(CO₂)–CuO pseudo-ternary system at 900 °C have revealed the existence of new indium–copper oxycarbonate – Ba₄In_0.8Cu_1.6(CO₃)_0.6O_6.2. Rietveld refinement of the X-ray powder diffraction data combined with infrared studies gives evidence that this phase is a oxycarbonate crystallising in the tetragonal structure (space group I4/mmm) with unit cell parameters: a=4.0349(1) Å and c=29.8408(15) Å. In the binary part of the In₂O₃–BaO(CO₂) system we have identified the occurrence of Ba₄In_2−x(CO₃)_1+xO_6−2.5x oxycarbonate solid solution showing a crystal structure also described by I4/mmm space group, but with the unit cell parameters: a=4.1669(1) Å and c=29.3841(11) Å for x=1. The existence range of this phase, −0.153<x<0.4, includes chemical compositions of earlier found phases: Ba₅In_2+xO_8+0.5x with 0≤x≤0.45 (known as the -solid solution), as well as the binary Ba₄In₂O₇ phase. The crystal structures of both new oxycarbonates are isomorphic and related to n=3 member of the Ruddlesden–Popper family.     

Magnetic properties of (Fe)1−x–(Al2O3)x and (Fe50Ni50)1−x–(Al2O3)x nanocomposite magnetic media synthesized using gel like Al2O3 matrix

Composites with ferromagnetic nanoparticles, Fe and Fe₅₀Ni₅₀, dispersed in Al₂O₃ have been synthesized by a solution phase technique. The structure and magnetic properties of these composites with varying fractions of Al₂O₃ have been investigated. Both Fe and Fe₅₀Ni₅₀ nanoparticles are amorphous in the as-prepared state and become crystalline on heat treating with near equilibrium lattice parameters of 0.287 nm and 0.358 nm respectively. The interparticle distance increases with increasing Al₂O₃ from 0 wt.% to 20 wt.%. The size of Fe nanoparticles is 40 nm while the Fe₅₀Ni₅₀ nanoparticles are 20 nm in size. The Fe and Fe₅₀Ni₅₀ nanoparticles dispersed composites are found to be ferromagnetic at room temperature both in the as-prepared and heat treated conditions with clear coercive fields of 5.5–35 × 10³ A m⁻¹. The saturation magnetization increases by orders of magnitude on heat treatment, for e.g. from <1.0 emu g⁻¹ to 143.4 emu g⁻¹ for Fe–15 wt.% Al₂O₃ and 95.6 emu g⁻¹ for Fe₅₀Ni₅₀–15 wt.% Al₂O₃. The Fe-composites exhibit a Curie transition at 1000 K while the Fe₅₀Ni₅₀ composites exhibit a transition at 880 K, both temperatures close to bulk values.     

Electrical, magnetic and elastic properties of La1.2(Sr1−xCax)1.8Mn2O7 (0.0 ≤ x ≤ 0.4)

Polycrystalline bulk samples of double layered manganite system La_1.2(Sr_1−xCa_x)_1.8Mn₂O₇ (0.0 ≤ x ≤ 0.4) were prepared by sol–gel method. After characterizing the samples using XRD and SEM, their electrical, magnetic and elastic properties were investigated. The lattice parameters and cell volume show a monotonous decrease with increase of Ca content, whereas the grain size is found to increase with increasing Ca content. The value of T_IM is found to decrease with Ca content up to x = 0.3 and then a slight increase of T_IM is observed. The low temperature upturn of resistivity is attributed to the spin-glass-like behavior, which is also evidenced by the irreversibility observed between ZFC and FC magnetizations. The conduction mechanism above T_IM can be explained by Mott VRH model. The present magnetization and ultrasonic studies indicate that the system shows a secondary transition at T^*, which decreases with increasing Ca content. Further, the T^* seems to be intrinsic to the present double layered manganite system.     

Structural, electrical and magnetic properties of Cu1−xZnxFe2O4 ferrites (0 ≤ x ≤ 1)

Copper–zinc ferrites bearing chemical formula Cu_1−xZn_xFe₂O₄ for x ranging from 0.0 to 1.0 with the step increment of 0.2 were prepared by the standard solid-state technique. The variation of Zn substitution has a significant effect on the structural, electrical and magnetic properties. Lattice parameters ‘a’ increased from 8.370 to 8.520 Å. Dielectric constant decreased up to 311 with the increase in frequency from 80 Hz to 1 MHz at room temperature. All the samples follow the Maxwell–Wagner's interfacial polarization. Saturation magnetization, magnetic moment and Yafet–Kittel angles were also determined. The possible reasons responsible for the change in density related, electrical and magnetic properties with the increase in Zn concentration are undertaken.     

Structural and magnetic properties of single-crystalline spinel systems ZnCr2−xAlxSe4 (x = 0.15 and 0.23)

Single crystals of the spinel solid solutions ZnCr_2−xAl_xSe₄ with x = 0.15 and 0.23 were grown and studied by X-ray diffraction and macroscopic magnetic measurements. The solubility of Al³⁺ ions in the parent compound ZnCr₂Se₄ is very limited and only weakly substituted single crystals can be obtained. The spinel structure is hardly modified by this admixture and regular cation distribution is preserved. Alike ZnCr₂Se₄, the two compounds investigated order antiferromagnetically at low temperatures. Both the Néel temperatures and the paramagnetic Curie temperatures are similar to that reported for the parent selenide. Also the effective magnetic moments are close to that of ZnCr₂Se₄ and compatible with trivalent Cr ions. In contrast, the saturation magnetic moments measured above the metamagnetic phase transitions in strong magnetic fields and calculated per one Cr atom, appear to be strongly affected by the Al-substitution, being rapidly suppressed with rising Al-content.     

Effect of Yb substitution on microstructure, physical and mechanical properties of negative thermal expansion Zr1−xYbxWMoO8−x/2 (x = 0–0.05) ceramic

Cubic Zr_1−xYb_xWMoO_8−x/2 (x = 0–0.05) ceramic was first fabricated by a polymorphous precursor transition method. X-ray diffraction experiment indicates that samples with x ≤ 0.05 are single phase solid solution. The measured bulk density, microstructure, maximal compression strength and Young's modulus are obviously sensitive to Yb substitution level, while none of such sensitivity was found for the lattice parameters, negative thermal expansion coefficients and Vickers hardness. Drilling tests on Zr_0.96Yb_0.04WMoO_7.98 ceramic indicate good machinability, which is often required for quality and shape control in engineering applications.     

Structural, electrical and magnetic properties of xNiFe2O4 + (1 − x)Ba0.8Sr0.2TiO3 ME composites

Magnetoelectric composites, namely xNiFe₂O₄ + (1 − x)Ba_0.8Sr_0.2TiO₃ were prepared by standard double sintering ceramic method. The X-ray diffraction measurements were carried out to check the phase purity and to calculate the lattice constants. scanning electron microscopy (SEM) micrographs were taken to understand the microstructure of the samples. Dielectric property such as dielectric constant (ε^′) was also studied as a function of frequency in the range 100 Hz–1 MHz. The ac conductivity as a function of frequency and dc resistivity as a function of temperature were studied for different compositions. The hysteresis measurements were done to determine saturation magnetization (M_s) and coersivity (H_c) and magnetic moment was calculated. Effect of resistivity on ME voltage coefficient is studied.     

In situ observation of thermal expansion of tetragonal C11b phase in Zr2Cu(1−x)Pdx alloys

The C11b phase crystalline structure (structure type MoSi₂, space group I4/mmm) in the Zr₂Cu_(1−x)Pd_x (x = 0, 0.25, 0.5, 0.75 and 1) alloys was examined in situ using high temperature X-ray diffraction (HTXRD) and Rietveld refinement of the data obtained at a constant heating rate. While the cell volume increases with increasing Pd as expected by the larger atomic radii, the coefficients of thermal expansion (CTEs) do not follow a uniform trend. The bonding in the basal plane is more elastically rigid than along the c-axis for all compositions. The CTE is more anisotropic for Zr₂Pd than for Zr₂Cu, which is consistent with the first-principles calculations that illustrate the rigidity of c-axis relatively to a-axis to be the less for Zr₂Pd. The CTE of the a-axis for Zr₂Pd is in fact negative over the temperature range measured.     

Oxygen release behaviour of Ce(1−x)ZrxO2 powders and appearance of Ce(8−4y)Zr4yO(14−δ) solid solution in the ZrO2–CeO2–CeO1.5 system

To clarify the existence of metastable phases in the ZrO₂–CeO₂–CeO_1.5 system, evolved-oxygen gas analyses, (EGA), by heating a single phase of t′ and t″ (Ce_(1−x)Zr_xO₂) with various compositions, x, in a reducing gas and successive oxidation were carried out repeatedly. The oxygen release behaviour of the t′ and t″ phases was very complicated. The single κ phases, (Ce_(1−x)Zr_xO₂) with the composition, x=0.5 and 0.6, which were obtained by oxidizing the resulting pyrochlore as a precursor in O₂ gas at 873 K, exhibited a sharp oxygen release at the lowest temperature; the composition range of κ phase may be x=0.450.65. A new tetragonal phase t*, (Ce_(1−x)Zr_xO₂), which was attained by cyclic redox process together with annealing in O₂ gas at 1323 or 1423 K, exhibited a sharp oxygen release at the highest temperature; the composition range of t* phase may be as wide as x=0.200.65. A metastable solid solution expressed by a chemical formula of Ce_(8−4y)Zr_4yO_(14−δ) (y=01) possessing a CaF₂-related structure appeared on deoxidation of the t* phase. A ternary phase diagram containing the t* and Ce_(8−4y)Zr_4yO_(14−δ) solid solution was proposed.