Synthesis under ambient pressure and tri-axial magnetic orientation in REBa2Cu4O8 (RE = Y,Sm, Eu,Gd, Dy,Ho, Er)

We report the rare-earth (RE)-dependent magnetization axes of REBa₂Cu₄O₈, which was synthesized by a flux method under ambient pressure, using powder samples tri-axially oriented in a modulated rotating magnetic field of 10 T. By optimizing the growth temperature and cooling rate, RE124 crystals were successfully grown for RE = Y, Sm, Eu, Gd, Dy, Ho, and Er. From the X-ray diffraction measurement, the magnetically oriented directions were largely dependent on the type of RE ions of RE124. However, the tri-axial magnetic anisotropies of RE124 could be qualitatively understood in terms of the magnitude relation between the single-ion magnetic anisotropy of RE³⁺ ions and the magnetic anisotropy generated by the CuO₂ plane and Cu–O chain. For the practical use of this magneto-scientific process, the control of magnetization axes and tri-axial magnetic anisotropies through crystallochemical control is indispensable.     

Oxide ion conduction mechanism in RE9.33(SiO4)6O2 and Sr2RE8(SiO4)6O2 (RE = La,Nd) from neutron powder diffraction

The oxide ion conduction mechanism was clarified for high purity RE_9.33(SiO₄)₆O₂ and Sr₂RE₈(SiO₄)₆O₂ (RE = La and Nd) by Rietveld and maximum entropy method (MEM) analysis using neutron powder diffraction data collected at room temperature. All the compounds had an apatite-type structure in the space group P6₃/m. Neither site splitting nor interstitial sites of the oxide ion was observed. RE_9.33(SiO₄)₆O₂ had cation vacancies only at the 4f site. In Sr₂RE₈(SiO₄)₆O₂, the 4f sites were fully occupied by strontium and rare earths with a molar ratio of 1 : 1. Also, in RE_9.33(SiO₄)₆O₂, the oxide ion at the hexagonal channel site had a large displacement along the c-axis. This large displacement is induced by cooperative rotation of SiO₄ tetrahedra around rare earths at the 4f site through oxide ion polyhedra around another rare earth at the 6h site. The displacement, enhanced by a vacancy in the 4f site, is directly related to oxide ion conduction in RE_9.33(SiO₄)₆O₂.     

(RE0.33Eu0.33Gd0.33)Ba2Cu3Oy bars processed in air

Bars of REBa₂Cu₃O_y superconductor with three different rare earths were prepared by floating zone technique in air. Their nominal composition is (RE_0.33Eu_0.33Gd_0.33)Ba₂Cu₃O_y (RE=Y, La, Pr, Nd, Sm and Yb). The mixing of these three elements leads to a single phase formation. The physical properties of these systems and their microstructure were studied. All the compositions exhibit a superconducting transition excepted that one containing Pr and substitution of Ba by RE is avoided excepted in the case of RE=La. Only the phase with Nd shows a pronounced fishtail effect and the phase with Sm exhibits a very high J_c value of 70 000 A/cm² in self-field and more than 30 000 A/cm² at 1.7 T.     

Anionic conductors Ln2/3[Bi12O14](MoO4)5 with Ln = La,Nd, Gd,Ho, Yb. Synthesis–spark plasma sintering–structure–electric properties

A new series of columnar phases Ln_2/3□_1/3[Bi₁₂O₁₄](MoO₄)₅ (□ vacancy) with Ln = La, Nd, Gd, Ho and Yb have been synthesized and structurally characterized. They feature the same formula as the molybdate phase Bi_2/3□_1/3[Bi₁₂O₁₄](MoO₄)₅ and crystallize in the monoclinic system, space group P2/c. These phases are isostructural with the prototype structure Bi[Bi₁₂O₁₄](MoO₄)₄(VO₄). Pellets of this rare-earth series obtained by spark plasma sintering and measured by impedance spectroscopy show a good anionic conductivity with a parabolic evolution whose maximum is raised by the Gd species at a determined value σ = 6.6 × 10^− 3 S cm^− 1 at 980 K.     

Anomalous Zn- and Ni-substitution effects on superconductivity in the superconducting weak ferromagnets RuSr2RCu2O8 (R = Gd,Eu)

The effect of magnetic Ni and non-magnetic Zn impurities on superconducting transition temperature T_c in RuSr₂R(Cu_1?x(Ni, Zn)_x)₂O₈ with R = Gd or Eu (Ni- and Zn-substituted Ru1212Gd(Eu)) was extensively studied. It is found that the suppression rate dT_c/dx of RuSr₂R(Cu_1?x(Ni, Zn)_x)₂O₈ is comparable to that of underdoped YBa₂(Cu_1?x(Ni, Zn)_x)₃O_7?δ. The suppression of superconductivity in Ni-substituted Ru1212Eu samples is more significant than that in Zn-substituted ones, indicative of Ni being a more effective pair-breaker than Zn. In strong contrast, the magnetic Ni impurity atoms have a weaker effect on superconductivity than non-magnetic Zn atoms in Ru1212Gd, similar to what was observed in the high-T_c cuprates. These intriguing findings strongly suggest that the impurity-induced local disturbance of the 3d-spin correlation at Cu sites around Ni/Zn is distinctly different between Ru1212Gd and Ru1212Eu.     

Effect of fluorine doping on phase formation and properties of Bi(Pb)-2223 ceramics

Superconducting ceramics of Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_yF_x (x = 0–0.6) are prepared in air by conventional solid state reaction and characterized. The study shows that the melting point of the samples decreases as fluorine content increases. As a consequence, the grain size increases with the doping level and for x = 0.6, the sample is completely deformed and presents a concave shape making impossible the measurements on it. The Vickers microhardness reaches its maximum for x = 0.2. The analysis of the X-ray diffraction results reveals that all the samples are composed of only Bi(Pb)-2212 and Bi(Pb)-2223 phases. The highest proportion of the high T_c phase (Bi(Pb)-2223) is also observed for x = 0.2 and is about 67.32%. The refinement of cell parameters is done by considering the structural modulation. The results show that the doping leads to a reduction of cell volume as well as the a axis component of modulation. From resistivity versus temperature measurements, it is shown that the doped phases exhibit higher onset critical transition temperatures than the undoped one. The residual resistivity increases with fluorine content suggesting that the doping introduces structural defects and disorder into the samples. The obtained critical current density at 77 K under zero magnetic field also increases with fluorine doping.     

Synthesis and visible light photocatalysis water splitting property of chromium-doped Bi4Ti3O12

Photocatalysts Bi₄Ti_3 ? xCr_xO₁₂(x = 0.00, 0.06, 0.15, 0.30, 0.40, and 0.50) with perovskite structure were synthesized by sol–gel method and their electronic structures and photocatalytic activities were investigated. The Bi₄Ti_2.6Cr_0.4O₁₂ photocatalyst exhibited the highest performance of H₂ evolution in methanol aqueous solution (58.1 μmol h^? 1 g^? 1) under visible light irradiation (λ > 400 nm) without a co-catalyst, whereas no H₂ evolution is observed for Bi₄Ti₃O₁₂ under the same conditions. The UV–vis spectra indicated that the Bi₄Ti_2.6Cr_0.4O₁₂ had strong photoabsorption in the visible light region. The results of density functional theory (DFT) calculation illuminate that the conduction bands of Bi₄Ti₃O₁₂ are mainly attributable to the Ti 3d + Bi 6p orbitals, and the valence bands are composed of O 2p + Bi 6s hybrid orbitals, while the conduction bands of chromium-doped Bi₄Ti₃O₁₂ are mainly attributable to the Ti 3d + Bi 2p + Cr 3d orbitals, and the O 2p + Cr 3d hybrid obitals are the main contribution to the valence band.     

Segregation at the surfaces of CuxPd1 − x alloys in the presence of adsorbed S

The influence of adsorbed S on surface segregation in Cu_xPd_1 ? x alloys (S/Cu_xPd_1 ? x) was characterized over a wide range of bulk alloy compositions (x = 0.05 to 0.95) using high-throughput Composition Spread Alloy Film (CSAF) sample libraries. Top-surface and near-surface compositions of the CSAFs were measured as functions of bulk Cu composition, x, and temperature using spatially resolved low energy ion scattering spectroscopy (LEISS) and X-ray photoemission spectroscopy (XPS). Preferential segregation of Cu to the top-surface of the S/Cu_xPd_1 ? x CSAF was observed at all bulk compositions, x, but the extent of Cu segregation to the S/Cu_xPd_1 ? x surface was lower than the Cu segregation to the surface of a clean Cu_xPd_1 ? x CSAF, clear evidence of an S-induced “segregation reversal.” The Langmuir–McLean formulation of the Gibbs isotherm was used to estimate the enthalpy and entropy of Cu segregation to the top-surface, ΔH_seg(x) and ΔS_seg(x), at saturation sulfur coverages. While Cu segregation to the top-surface of the clean Cu_xPd_1 ? x is exothermic (ΔH_seg < 0) for all bulk Cu compositions, it is endothermic (ΔH_seg > 0) for S/Cu_xPd_1 ? x. Segregation to the S/Cu_xPd_1 ? x surface is driven by entropy. Changes in segregation patterns that occur upon adsorption of S onto Cu_xPd_1 ? x appear to be related to formation of energetically favored PdS bonds at the surface, which counterbalance the enthalpic driving forces for Cu segregation to the clean surface.     

Structural and magnetic phase transitions of kagome-like compounds REBaCo4O7 (RE=Dy,Ho, Er,Tm, Yb,Lu)

In a temperature range 5–300 K the specific heat C(T) on a new mixed valence cobalt oxides REBaCo₄O₇ (RE=Dy, Ho, Er, Tm, Yb, Lu) was investigated. The first-order structural phase transitions from hexagonal P6₃mc to orthorhombic Cmc₂1 phase was indicated by a peak-like anomaly in C(T) curves at T_S～160, 178, 224, and 280 K for RE=Lu, Yb, Tm, and Er correspondingly. The magnetic phase transitions was indicated as the changes of slope on the C(T) curves were found at corresponding temperatures: T_N～50, 74, 98, and 98 K for RE=Lu, Yb, Tm, and Er, correspondingly.     

Influence of Cu doping on the microstructure,optical properties and photoluminescence features of Cd0.9Zn0.1S nanoparticles

Cd_0.9−xZn_0.1Cu_xS (0≤x≤0.06) nanoparticles were successfully synthesized by a conventional chemical co-precipitation method at room temperature. Crystalline phases and optical absorption of the nanoparticles have been studied by X-ray diffraction (XRD) and UV–visible spectrophotometer. XRD confirms the phase singularity of the synthesized material, which also confirmed the formation of Cd–Zn–Cu–S alloy nanocrystals rather than separate nucleation or phase formation. Elemental composition was examined by the energy dispersive X-ray analysis and the microstructure was examined by scanning electron microscope. The blue shift of absorption edge below Cu=2% is responsible for dominance of Cu⁺ while at higher Cu concentration dominated Cu²⁺, d–d transition may exist. It is suggested that the addition of third metal ion (Cu²⁺/Cu⁺) is an effective way to improve the optical property and stability of the Cd_0.9Zn_0.1S solid solutions. When Cu is introduced, stretching of Cd–Zn–Cu–S bond is shifted lower wave number side from 678 cm⁻¹ (Cu=0%) to 671 cm⁻¹ (Cu=6%) due to the presence of Cu in Cd–Zn–S lattice and also the size effect. The variation in blue band emission peak from 456 nm (∼2.72 eV) to 482 nm (∼2.58 eV) by Cu-doping is corresponding to the inter-band radiation combination of photo-generated electrons and holes. Intensity of red band emission centered at 656 nm significantly increased up to Cu=4%; beyond 4% it is decreased due to the quenching of Cu concentration.     

Infrared-to-visible up-conversion luminescence and energy transfer of RE3+/Yb3+(RE = Ho,Tm) co-doped SrIn2O4

Near-infrared excited up-conversion phosphors of RE³⁺/Yb³⁺(RE = Ho, Tm) co-doped SrIn₂O₄ were synthesized by a solid-state reaction method. X-ray diffraction analysis revealed the phase composition of those samples, and the up-conversion spectroscopic properties were studied in terms of up-conversion emission spectra. Under 980 nm near-infrared laser excitation, strong green emission with the peak at 546 nm was observed in SrIn₂O₄: Ho³⁺/Yb³⁺, which can be assigned to the characteristic ⁵S₂(⁵F₄) → ⁵I₈ transition of Ho³⁺. Furthermore, SrIn₂O₄: Tm³⁺/Yb³⁺ showed bright blue emission with the peak at 486 nm, which is associated with the ¹G₄ → ³H₆ transition of Tm³⁺. The UC power studies indicated that the luminescence of SrIn₂O₄: Ho³⁺/Yb³⁺ and SrIn₂O₄: Tm³⁺/Yb³⁺ are attributed to two-photon and three-photon process, respectively. The possible UC luminescence mechanism and energy transfer in SrIn₂O₄: RE³⁺/Yb³⁺ were discussed.     

Magnetoresistance hysteresis of bulk textured Bi1.8Pb0.3Sr1.9Ca2Cu3Ox + Ag ceramics and its anisotropy

Magnetoresistance of bulk textured Bi_1.8Pb_0.3Sr_1.9Ca₂Cu₃O_x + Ag ceramics has been studied in the magnetic fields applied parallel and perpendicular to a–b planes of Bi2223 crystallites. Besides well known anisotropy of magnetoresistance of textured superconductors (R_H || c > R_H || a–b), anisotropic hysteresis of R(H) dependences was investigated. Parameters characterizing hysteretic R(H) curves differ for the cases H || c and H || a–b. This behavior is explained within the model of a granular superconductor where the total magnetic induction in the intercrystallite boundaries is superposition of the external field and the magnetic field induced by dipole magnetic moments of neighbor crystallites.     

In situ characterization of transport properties of superconducting (Cu, C)-system thin films

Transport properties of (Cu, C)Ba₂CuO_x [(Cu, C)-1201] thin films have been characterized by in situ temperature dependence of resistivity, without breaking vacuum from the deposition to the measurement. In in situ transport properties measurements, the obtained results reveal that (Cu, C)Ba₂CuO_x films exhibit T_c > 20 K on the cased of conductivity at 290 K (σ_[290 K]) > 4 × 10² S/cm and temperature coefficient of resistivity (TCR) > 1.5 × 10^?3 K^?1, and doping level of them should be in between under-doped and optimally-doped states. Their results suggest that there would be possible to further increases of T_c, and XPS data suggest that (Cu, C)-system should have the excellent dopability in their charge reservoir and the possibility of low anisotropy.     

Electrochemical and magnetic properties of Li4 / 3Mn2 / 3O2–LiCoO2 solid solution

The compounds Li_(4−x)/3Mn_2(1−x)/3Co_xO₂ (0 < x < 0.5) were prepared by the sol–gel technique. X-ray diffraction patterns of these compounds were identified as α-NaFeO₂ type layered structure, though some super-structure lines, related to the ordered array of Li and transition metal ions in the transition metal layer, were observed. The magnetic susceptibility exhibited an antiferromagnetic transition around 40 K for x < 0.2, however the specimens with x > 0.3 had no magnetic transition. The magnetic percolation may explain these magnetic variations. The electrochemical performances were evaluated for the compound of x = 0.5, and it was seen that the electrochemical properties were sensitive to the potential window. Additionally, it was also found that the discharge capacity strongly depended on the preparation temperature; it took a maximum value at the preparation temperature of 900 °C. The discharge capacity is sensitive not only to the cation mixing degree but also to the particle size.     

Use of film thickness and Cu additive to improve (0 0 1) texture in MgO/FePtCu(C) bilayers

A multilayer structure has been proposed that demonstrates improved (0 0 1) texture for FePt-based L1₀ perpendicular media. Achieving a strong perpendicular magnetic anisotropy requires aligning the L1₀ crystallographic c-axis along the film normal. The ordered L1₀ FePt structure is tetragonal with a c/a ratio close to 0.965. This makes discriminating between the three crystallographic variants ([1 0 0], [0 1 0], and the desired [0 0 1]) difficult. Alloying FePt with Cu to reduce the c/a ratio and using a multilayer approach to keep the magnetic layers thin results in a structure with an adjustable M_rt and a strong (0 0 1) texture (rocking curve widths around 2°). This is a remarkable improvement in texture from pure FePt multilayered films or monolithic FePt(X) films. The proposed [MgO(2 nm)/Fe_50−xPt₅₀Cu_x(5 nm)]_×n structure limits grain size in the vertical (perpendicular) direction albeit not in the plane of the film. Carbon can be added to the FePtCu layer to reduce the grain size with minimal degradation of the (0 0 1) orientation.     

Crystal chemistry and physical properties of the ternary compounds RE5B4C5 (RE=Ce,Pr, Nd)

The ternary rare-earth metal boride carbides RE₅B₄C₅ (RE=Ce, Pr, Nd) were prepared by arc melting the mixtures of the pure elements and subsequent annealing at 1270 K. Their crystal structures were determined from single crystal X-ray diffraction data. They crystallize in the Ce₅B₄C₅ type of structure (space group Pna2₁, Z=8). Two new compounds were found, Pr₅B₄C₅: a=24.592(2) Å, b=8.4563(5) Å, c=8.4918(5) Å, R₁=0.043 (wR₂=0.076) for 2871 reflections with I_o?2σ(I_o)); for Nd₅B₄C₅: a=24.301(1) Å, b=8.3126(5) Å, c=8.3545(4) Å, R₁=0.035 (wR₂=0.069) for 3707 reflections with I_o?2σ(I_o)). Its structural arrangement consists of a three-dimensional framework of rare-earth atoms resulting from the stacking of slightly corrugated two-dimensional square nets, leading to voids filled with B₄C₄, B₃C₃, BC₂ finite chains and isolated carbon atoms. Ce₅B₄C₅ is an antiferromagnet at 5 K followed by a metamagnetic transition in elevated external fields. Pr₅B₄C₅ and Nd₅B₄C₅ are ferromagnets below T_C=12 and 15 K, respectively.     

Luminescence and evaluation of trapping parameters in NaMgSO4Cl: X (X=Cu or Ce) phosphor

Thermoluminescence (TL) characteristics of recently developed high sensitive mixed halosulphate phosphors, NaMgSO₄Cl: Cu and NaMgSO₄Cl: Ce were studied in comparison with CaSO₄: Dy in order to assess the possibility of their use in personal monitoring and TLD phosphors at very low dose of 5 Gy. It was found that NaMgSO₄Cl: Cu is 5.59 times and NaMgSO₄Cl: Ce is 6.18 times more sensitive as compared to standard CaSO₄: Dy. UV photo-excited luminescence from Cu to Ce doped NaMgSO₄Cl halosulphate phosphors has been investigated. The intense emission of the spectrum is assigned to electronic transitions 3d⁹4s¹→3d¹⁰ in monovalent copper ion and 5d→4f in Ce³⁺ ions. Increase in PL peak intensity suggesting that Cu and Ce play an important role in PL emission in the present matrix. These phosphors were synthesized by the wet chemical method. XRD, photoluminescence (PL) and thermoluminescence (TL) characterization of phosphors has been reported in this paper. The preparation of an inexpensive and high sensitive NaMgSO₄Cl: Cu and NaMgSO₄Cl: Ce with TL glow peaks for different concentrations are observed between 160 and 195 °C and between 200 and 225 °C, respectively, exposed to gamma-rays of ⁶⁰Co for their thermoluminescence (TL) properties. The glow curves have been recorded at a heating rate of 2 K s^?1 and irradiated at a dose rate of 0.995 kGy h^?1 for 5 Gy. In present study the trapping parameters such as order of kinetics (b), activation energy (E) and frequency factors (s) have been calculated for the 195 and 200 °C glow peaks of NaMgSO₄Cl: Cu and NaMgSO₄Cl: Ce, respectively by using Chen's method. The paper discusses the luminescence of Cu⁺ and Ce³⁺ by simple method of incorporation in NaMgSO₄Cl host.     

Effect of oxygen gas pressure on orientations of Cu2O nuclei during the initial oxidation of Cu(100), (110) and (111)

The orientations of oxide nuclei during the oxidation of Cu(100), (110) and (111) surfaces have been examined by in situ transmission electron microscopy. Our results indicate that the epitaxial nucleation of oxide islands on these surfaces cannot be maintained for a whole range of oxygen gas pressure varying from 10^? 5 Torr to 750 Torr. The critical oxygen gas pressure, pO₂, leading to the transition from nucleating epitaxial to non-epitaxial oxide nuclei shows a dependence on the crystallographic orientations of the Cu substrates with pO₂⁽¹⁰⁰⁾ > pO₂⁽¹¹¹⁾ > pO₂⁽¹¹⁰⁾. By fitting the experimentally determined critical oxygen pressures to a kinetic model, we find that such dependence can be attributed to the effect of surface orientations of the Cu substrates on the oxygen surface adsorption and diffusion, which dominate the kinetic processes of oxide nucleation.     

Stoichiometry and surface reconstruction of epitaxial CuInSe2(112) films

CuInSe₂(112) films were grown on GaAs(111)A substrates by molecular beam epitaxy. The resulting surface stoichiometry was deduced by consideration of results from various surface analytic techniques. The obtainable Cu/In stoichiometry range in XPS was 0.4–1.2, where 1.2 marks the onset of Cu_2 ? xSe phase segregation at the surface and 0.4 corresponds to the copper-depleted surface with ordered defect compound (ODC) composition. For the stoichiometric CuInSe₂(112) surface, a c(4 × 2) reconstruction of the zinc blende surface periodicity is observed in the LEED pattern, with three rotational domains present on the flat GaAs(111) substrate. With the use of stepped (111) substrates, domain formation could be suppressed. By comparison of the LEED data and concentration depth profiles from angle-resolved XPS, two types of surface reconstructions could be distinguished. According to surface energy calculations in the literature, these correspond to surfaces stabilized by either Cu_In or 2V_Cu defects. The surface of copper-poor CuIn₃Se₅ shows no reconstruction of the zinc blende order.     

Fabrication of single domain GdBCO bulks with different new kind of liquid sources by TSIG technique

Single domain GdBCO bulk superconductors have been fabricated with different new kinds of liquid sources by Top Seeded Infiltration and Growth process. A conventional liquid source is a mixture of REBa₂Cu₃O_7?x and Ba₃Cu₅O₈ in a ratio of 1:1; new liquid sources are made of a mixture of RE₂BaCuO₅ + 9BaCuO₂ + 6CuO or RE₂O₃ + 10BaCuO₂ + 6CuO. The microstructures and levitation forces of the GdBCO bulks have also been investigated. The properties of the samples fabricated are nearly the same as those of samples fabricated with conventional liquid sources, which indicates that the new liquid sources are very important to reduce the cost and improve the working efficient on the fabrication of single-domain GdBCO bulk superconductors.