Magnetic Anomalies and Electronic Structure of Ce<Subscript>2</Subscript>Cu<Subscript>2</Subscript>Mg and Ce<Subscript>2</Subscript>Pd<Subscript>2</Subscript>Mg

The intermetallic compounds RE ₂Cu₂Mg and RE ₂Pd₂Mg (RE=La, Ce) were prepared and characterized by magnetic susceptibility and heat-capacity measurements. They crystallize with an ordered U₃Si₂ type structure (space group P4/mbm) and the cerium compounds contain stable trivalent cerium ions, which undergo antiferromagnetic ordering at ∼7.5 and ∼4 K, respectively. In high magnetic fields Ce₂Cu₂Mg exhibits spin reorientation with critical field strength of 40 kOe at 5 K. The data are compared to the non-magnetic isotypic compounds La₂ T ₂Mg (T=Cu, Ni, Pd) and to the already reported intermediate valent Ce₂Ni₂Mg.     

The Influence of Neutron Irradiation on the Critical Temperature and Normal Phase Resistance of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">δ</Emphasis></Subscript> High Temperature Superconductors

The variation of the normal phase resistance, transition width and critical temperature T _c by the irradiation bulk samples of Y123 with different doses of neutrons at the temperature range of 70<T<300 K has been investigated comparatively. The nonirradiated sample shows a superconducting transition temperature of T _c=86 K (zero-resistance temperature). The irradiation produced clear enhancement of normal phase resistance at all investigated temperatures, which provides the evidence for the enhancement of residual resistance, so the results of experiments showed that the transition width and critical temperature of specimens varies nonlinearly with increasing irradiation fluencies. These results are explained in terms of oxygen declension, and some defects induced by neutrons.      

Effect of Non-Stoichiometry Oxygen Content on the Magnetization of La<Subscript>1.5</Subscript>Sr<Subscript>0.5</Subscript>NiO<Subscript>4+<Emphasis Type="Italic">δ</Emphasis></Subscript>

We report on magnetization measurements of La_1.5Sr_0.5NiO_4+δ for δ=0.01 and δ=−0.01. Zero field-cooled and field-cooled bulk magnetization measurements were performed parallel to the Ni–O planes of single crystals, and on a polycrystalline δ=0.01 sample. Striking differences in the magnetization curves are observed between the two doping levels that are identifiable in polycrystalline δ=0.01. The bulk magnetization differences indicate a crossover in the magnetic properties of La_2−x Sr _x NiO_4+δ at half doping level, which can be used as a simple non-destructive way to determine whether La_1.5Sr_0.5NiO_4+δ is under or over half doping.     

High-temperature tensile-hold crack-growth behavior of HASTELLOY<Superscript>®</Superscript> X alloy compared to HAYNES<Superscript>®</Superscript> 188 and HAYNES<Superscript>®</Superscript> 230<Superscript>®</Superscript> alloys

The creep–fatigue crack-growth tests of HASTELLOY^® X alloy were carried out at the temperatures of 649°C, 816°C, and 927°C in laboratory air. The experiments were conducted under a constant stress-intensity-factor-range (ΔK) control mode with a R-ratio of 0.05. In the constant ΔK tests, a ΔK of 27.5 MPa\(\sqrt{\mathrm{m}}\) and a triangular waveform with a frequency of 0.333 Hz were used. Various tensile hold times at the maximum load were imposed to study fatigue and creep–fatigue interactions. Crack lengths were measured by a direct current potential drop method. In this paper, effects of hold time and temperature on the crack-growth rates are discussed. Furthermore, the crack-growth rates of the HASTELLOY^® X alloy are compared to those of the HAYNES^® 188 and HAYNES^® 230^® superalloys.     

Calculated Sound Velocity Change in LaFeAsO<Subscript>0.89</Subscript>F<Subscript>0.11</Subscript> at the Superconducting Transition

The changes in the bulk modulus and sound velocity in LaFeAsO_0.89F_0.11 superconductor at the transition temperature were calculated using various thermodynamic data. Step discontinuities of 16 ppm in the bulk modulus and 5 ppm in the longitudinal sound velocity are expected at the superconducting transition. The calculated acoustic Debye temperature for the parent compound is 367 K and this is slightly higher than the values determined by specific heat measurements for LaO_0.89F_0.11FeAs.     

Study of the Influence of MgO Nano-Oxide Addition on the Electrical and Mechanical Properties of (Cu<Subscript>0.25</Subscript>Tl<Subscript>0.75</Subscript>)-1234 Superconducting Phase

The effect of nano-size MgO (40–60 nm) addition on the physical properties of polycrystalline (Cu_0.25Tl_0.75)-1234 was studied. The MgO-concentration (x) varied from 0.0 to 1.0 wt.% of the sample’s total mass. X-ray analysis, granular structure examination, energy dispersive X-ray spectroscopy, electrical resistivity, critical current density at different applied magnetic fields and Vickers microhardness measurements were carried out to study the performance of these samples. Phase examination by X-ray diffraction indicated that MgO-addition enhanced the (Cu_0.25Tl_0.75)-1234 phase formation till x=0.6 wt.%. Granular investigation by scanning electron microscope reveled that both number and size of voids decreased as x increased from 0.0 to 0.6 wt.%. These results were supported by the sample’s porosity calculation. The superconducting transition temperature and critical current density were found to have optimum values at x=0.6 wt.%. Furthermore, the Vickers microhardness increased till x=0.6 after that it decreased with further increase in x.     

Electronic Structure of BaFe<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>As<Subscript>2</Subscript> Revealed by Angle-Resolved Photoemission Spectroscopy

We report an angle-resolved photoemission spectroscopy study of BaFe_2−x Co _x As₂. For x=0, above the structural and magnetic transition temperature (T _s ), the spectral weight near the Fermi level is considerably suppressed around the Γ point where the Fe 3d yz/zx orbital degeneracy is expected. This observation suggests that the Jahn–Teller type instability is playing an important role in the tetragonal phase above T _s . Below T _s , the spectral weight of 0–100 meV is reconstructed to form flat bands at 70–100 meV and Fermi surfaces, consistent with the orbital-dependent excitonic coupling. In the optimally doped and overdoped regimes, the hole pocket around the Γ point and the electron pocket around the M point are apparently nested, indicating that the doping dependence of the superconducting transition temperature cannot be explained by the nesting scenario and that the unusual electron–lattice fluctuation due to the orbital degeneracy is important.     

Facile Synthesis of Co<Subscript>3</Subscript>O<Subscript>4</Subscript>/CoO Nanoparticles by Thermal Treatment of Ball-Milled Precursors

Co₃O₄/CoO nanoparticles have been synthesized by a simple method which is based on the ball-milling and calcination of cobalt acetate and citric acid. The samples were characterized using X-ray diffraction, transmission electron microscope, and Fourier transform infrared spectroscopy. The results show that Co₃O₄ nanoparticles with an average particle size of ∼40 nm can be obtained by calcination of ball-milled precursors at relatively low temperature (350 °C) for 3 hours. It should be noted that it is possible to control the size of Co₃O₄ particles by calcination temperature, calcination time and also by ball-milling duration using this method. Meanwhile, the pure CoO nanoparticles were obtained successfully by thermal decomposition of Co₃O₄ at 950 °C and quickly quenching to liquid nitrogen.     

Superconducting Bands Stabilizing Superconductivity in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7</Subscript> and MgB<Subscript>2</Subscript>

It is shown that the ceramic superconductor YBa₂Cu₃O₇ as well as the superconducting intermetallic compound MgB₂ possesses a narrow, partly filled “superconducting band” with Wannier functions of special symmetry in their band structures. This result corroborates previous observations about the band structures of numerous superconductors and non-superconductors showing that evidently superconductivity is always connected with such superconducting bands. These findings are interpreted in the framework of a nonadiabatic extension of the Heisenberg model. Within this new group-theoretical model of correlated systems, Cooper pairs are stabilized by a nonadiabatic mechanism of constraining forces effective in narrow superconducting bands. The formation of Cooper pairs in a superconducting band is mediated by the energetically lowest boson excitations in the considered material that carry the crystal-spin angular momentum 1⋅ℏ. These crystal-spin-1 bosons are proposed to determine whether the material is a conventional low-T _c or a high-T _c superconductor. This interpretation provides the electron–phonon mechanism that enters the BCS theory in conventional superconductors.     

Ionic conductivity ageing behaviour of 10 mol.% Sc<Subscript>2</Subscript>O<Subscript>3</Subscript>–1 mol.% CeO<Subscript>2</Subscript>–ZrO<Subscript>2</Subscript> ceramics

The long-term ionic conductivity behaviour of samples of zirconia co-doped with 10 mol.% of Sc₂O₃ and 1 mol.% CeO₂ is evaluated in oxidizing and reducing atmospheres at 600 °C. After 3,000 h, the sample kept in reducing atmospheres exhibits 20% loss in the ionic conductivity, while the sample kept in air shows 6% degradation. No phase transitions were observed in the samples after both the ageing studies. The main contribution towards the loss in the ionic conductivity of the sample kept in air comes from grain boundaries; however, for the sample aged in reducing conditions, both grain and grain boundary contribute similarly towards the increase in the total resistivity. This is tentatively explained by the reduction of Ce⁴⁺ cations, dissolved in the fluorite lattice of ZrO₂.     

Stability of the LaCl<Stack><Subscript>4</Subscript><Superscript>−</Superscript></Stack> and LuCl<Stack><Subscript>4</Subscript><Superscript>−</Superscript></Stack> Ions Studied by Mass Spectrometry

The enthalpy stability of the LaCl ₄ ^? and LuCl ₄ ^? ions is assessed using high-temperature mass spectrometry. The enthalpy of Cl^? detachment is determined to be Δ_rH⁰(298.15 K) = 332 ± 10 kJ/mol for LaCl ₄ ^? and 359 ± 10 kJ/mol for LuCl ₄ ^? .     

1.06 μm laser characteristics of Nd:KY(WO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystal

This paper reported the pulse laser characteristics of Nd³⁺:KY(WO₄)₂ crystal grown in our laboratory with an impurity of 4.57%. The maximum laser output was 61.6 mJ at 1.06 m, pumped by a single Xenon flash-lamp. The laser energy threshold value E_th was 0.32 J. The overall and slope efficiencies were 0.43% and 0.45%.     

Increased Solubility of Mg at Ca Site in Cu<Subscript>0.5</Subscript>Tl<Subscript>0.5</Subscript>Ba<Subscript>2</Subscript>Ca<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>10−<Emphasis Type="Italic">δ</Emphasis></Subscript> Superconductor

The Cu_0.5Tl_0.5Ba₂Ca_2?y Mg _y Cu₃O_10?δ (y=0, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0, 1.5, 2.0) superconductor has been synthesized at the atmospheric pressure by the solid-state reaction method. The zero resistivity critical temperature is found to increase to 98 K with Mg concentration of y=0.6, but saturates to 97 K with further enhancement of Mg to y=0.8, 1.0, and 1.5. The Mg doped material grows in tetragonal structure and follows P4/mmm symmetry with a &; c-axes lengths of 3.894 Å &; 15.091 Å for y=1.5. The axes lengths were decreased with the increase of Mg content in the unit cell, which shows that anisotropy of the material decreases. The critical current density and the quantity of diamagnetism in the samples with Mg contents are higher than in the samples without Mg. In order to realize the effects of decreased axes lengths on the phonon modes of Cu_0.5Tl_0.5Ba₂Ca_2?y Mg _y Cu₃O_10?δ , we have carried out FTIR absorption measurements.     

Formation-decomposition equilibrium of the NpO<Stack><Subscript>2</Subscript><Superscript>+</Superscript></Stack>·UO<Stack><Subscript>2</Subscript><Superscript>2+</Superscript></Stack> complex in chloride melts

The absorption spectra of the NpO ₂ ⁺ (5f ²) ion were examined in the region of the 3H ₅ ← ³ H ₄ magnetic dipole transition (1530–1760 nm) for series of melts with the UO ₂ ²⁺ concentration varied in the opposite directions: (1) NaCl-2CsCl eutectic melt with growing additions of the Cs₂UO₂Cl₄ complex salt and (2) Cs₂UO₂Cl₄ melt with growing additions of the NaCl-2CsCl mixture. Measurements of the integrated intensities of the bands belonging to the NpO ₂ ⁺ ·UO ₂ ²⁺ complex and unbound NpO ₂ ⁺ throughout the UO ₂ ²⁺ concentration range examined (up to 4.4 M in neat Cs₂UO₂Cl₄ melt) and processing of the data obtained in terms of the mass action law showed that the formation-decomposition reaction of the cation-cation complex can be described adequately only using the equation of reaction in the form NpO₂Cl ₄ ^3? + UO₂Cl ₄ ^2? ? {Cl₄ONpO?UO₂Cl₃}^4? = Cl^? (with the equilibrium constant of 1.3±0.1). Thus, the formation of the cationcation complex should be treated as replacement of chloride ion in the equatorial plane of uranyl(VI) by neptunyl(V), rather than as simple addition of UO ₂ ²⁺ to NpO ₂ ⁺ . The reverse reaction, decomposition of the cation-cation complex, consists essentially in replacement of neptunyl(V) by chloride ion.     

Superconductivity at 57.3 K in La-Doped Iron-Based Layered Compound Sm<Subscript>0.95</Subscript>La<Subscript>0.05</Subscript>O<Subscript>0.85</Subscript>F<Subscript>0.15</Subscript>FeAs

Here, we report the synthesis and super- conductivity of the La-doped iron-based compound Sm_0.95La_0.05O_0.85F_0.15FeAs; the resistivity and magnetic measurement indicate that the onset transition temperature as high as 57.3 K. This compound has the same crystal structure as ZrCuSiAs type tetragonal layered structure. The higher T _c compares to SmO_0.85F_0.15FeAs was attributed to partly La-substitution.     

Estimating the effect of C<Stack><Subscript>2</Subscript><Superscript>−</Superscript></Stack> and C<Stack><Subscript>3</Subscript><Superscript>+</Superscript></Stack> radicals and neutral clusters on the fullerene formation

The mass spectrum of the products of arc discharge in helium between graphite electrodes has been studied for various values of the gas flow rate. As the gas flow rate increases, the intensity of C₆₀^±, C₇₀^±, C₈₄^± and C₉₀^± fullerene peaks increases and that of the C₂⁻ and C₃⁺ cluster radicals decreases, but the total decay in radicals amounts to only 21% of the total growth of fullerenes. From this it follows that a contribution to the formation of fullerenes from the neutral clusters (which are taken into account for the first time) significantly exceeds the contribution due to small radical species.     

Preparation and structural study of Mg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>TiO<Subscript>3</Subscript> ceramics and their dielectric properties from 1 Hz to 7.7 GHz

A series of Mg_1?x Zn _x TiO₃, x = 0–0.5 (MZT0–MZT0.5) ceramics was synthesised and characterised. The dielectric properties of the samples in the frequency range of 1 Hz–7.7 GHz were explored using three different methods: a contacting electrode method, a parallel-plate method and a perturbed resonator method. The electrical properties in the space charge and dipolar polarisation frequency ranges are discussed in relation to the phase composition and microstructure data. Differences in the zinc substitution divided the dielectrics into two groups, namely MZT0–MZT0.2 and MZT0.3–MZT0.5, each with different amount of a main Mg_1?x Zn _x TiO₃ solid solution phase and a secondary solid solution phase. Zinc substitution promoted the density of the ceramics, improved the purity of the main phase and increased the permittivity for frequencies up to 10⁸ Hz, but reduced the permittivity in the microwave range. In the MZT0.3–MZT0.5 samples, for frequencies less than 1 MHz the quality (Q × f) factors were lower and log σ _a.c, the AC conductivity, was higher than for the MZT0–MZT0.2 samples. Above 10 MHz, the (Q × f) factors and log σ _a.c of the two groups were similar.     

Impact of Particle Size on Room Temperature Ferrimagnetism of SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript>

In this paper, we report the control of important hysteresis parameters, which are useful for memory devices, viz. M _s , H _c and M _r /M _s , by changing the particle size/calcination temperature. An investigation of SrFe₁₂O₁₉ nanopowder from the structural and magnetic aspect is performed using X-ray diffraction (XRD), High Resolution Transmission Electron Microscopy (HRTEM), Scanning Electron Microscopy (SEM) and Vibrating Sample Magnetometer (VSM). The average particle size (APS) of SrFe₁₂O₁₉(nanopowder) increases from 26 to 600 nm with calcination temperatures of 400 and 1100 °C in air, respectively. With the increase in calcination temperature, saturation magnetization (M _s ) increases with the decrease in coercivity for the respective sample. The change in saturation magnetization and coercive field are explained on the basis of transition from single domain structure to multi-domain geometry with an increase in the heating temperature. The sample heated at 1000 °C shows a minimum coercive field (2.71 kOe) and an appropriate squareness ratio (M _r /M _s ) compared to other calcined samples.     

The reaction Pu(VI) + Fe(CN)<Stack><Subscript>6</Subscript><Superscript>3−</Superscript></Stack> ⇄ Pu(VII) + Fe(CN)<Stack><Subscript>6</Subscript><Superscript>4−</Superscript></Stack> in NaOH solutions

A spectrophotometric study showed that, in 5 M NaOH, Pu(VII) prepared by ozonation of Pu(VI) is reduced with excess K₄Fe(CN)₆. The Pu(VII) content can be estimated from the amount of the Fe(CN)₆³⁻ formed. In NaOH solutions of concentration exceeding 8 M, the Fe(CN)₆³⁻ ion oxidizes Pu(VI). In 10.3 M NaOH, the tenfold excess of K₃Fe(CN)₆ fully converts 1 mM Pu(VI) to the heptavalent state within 4 min (rate constant 1.3 l mol⁻¹ s⁻¹ at 20°C). With an increase in the NaOH concentration, the oxidation rate increases, and smaller excess of K₃Fe(CN)₆ is required. This oxidant is consumed not only for Pu(VI) oxidation but also in reactions with H₂O and OH⁻ ions. Pu(VII) is unstable and is slowly reduced with water and with products of decomposition of iron complexes.     

Investigations on the Optical Spectra and EPR Parameters for Tetragonal Sm<Superscript>3+</Superscript> Center in SmBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">δ</Emphasis></Subscript>

It is of interest to study the optical spectra and EPR parameter of the rare-earth ion in the typical high T _c superconductors. The proper interpretation for these values is helpful to understand the optical and magnetic characteristics of the superconductors. In this paper, we calculate the optical spectra and EPR parameters (g factors g _∥ ,g _⊥ and hyperfine structure constants A _∥ , A _⊥ ) of tetragonal Sm³⁺ center in YBa₂Cu₃O_7−δ from the perturbation formulas of EPR parameters for 4f⁵ ion in tetragonal symmetry. In these formulas, the crystal-field J-mixing of the first and second excited-state multiplets ⁶H_7/2 and ⁶H_9/2 into the ground state, ⁶H_5/2, the mixtures among the states with the same J value via spin-orbit coupling interaction and the interactions between the ground Kramers doublet Γ γ and the same irreducible representation as Γ γ in other 11 Kramers doublets Γ x within ⁶H _J (J=5/2, 7/2, 9/2) states via crystal field and orbital angular momentum (or hyperfine structure) are considered. The calculated results are in reasonable agreement with the observed values. The results are discussed.