Preparation, characterization and oxide ion conductivity in U-substituted Bi4V2O11

A Bi₂V_{1 − x − y}U_xBi_yO_{5.5 + 0.5x − y} solid solution derived from Bi₄V₂O₁₁ has been prepared and characterized with x up to 0.125 for y = 0. Partial substitution of U⁶⁺ for V⁵⁺ in Bi₄V₂O₁₁ leads to the stabilization at room temperature of the high-oxide ion conducting γ-phase, in contrast with other M⁶⁺ dopants which stabilize the β-phase. The lower conductivity in U substituted system compared with BICUVOX.10 is attributed to its higher activation energy. Conductivity values and activation energies of the U substituted phases compare well with Bi₂UO₆.     

Tuning the multiferroic properties of Pb(Fe1/2Nb1/2)O3 by cationic substitution

A series of substituted lead iron niobate compounds with the general formula Pb²⁺_(1−x)A^Z_x(Fe_{{(1−(2−Z)x)/2}}Nb_{{(1+(2−Z)x)/2}})O₃ (0<x<0.6 and A=La³⁺, K⁺ or Sr²⁺) were prepared by a modified solid-state synthesis. The relative concentrations of Fe³⁺ and Nb⁵⁺ were adjusted to compensate the charge imbalance due to the aliovalent substitution. The dielectric constant and magnetic susceptibilities were studied as a function of temperature. The temperature of the dielectric maximum, T_M, of the substituted compounds decreased linearly with increasing concentration of the substituent ions. The magnetic measurements showed an antiferromagnetic transition at temperatures T_N1 due to the superexchange interactions mediated by Fe–O–Fe and an additional antiferromagnetic-type transition at T_N2. T_N1 linearly increased with the increasing concentration of Fe³⁺ ion at the B-site of ABO₃-type substituted compounds. T_M is shown to be directly dependent on the concentration of the ferroactive Nb⁵⁺ ions at the B-site and Pb²⁺ ions at the A-site.     

Shaped crystal growth and scintillating properties of Yb:(Gd,Lu)3Ga5O12 solid solutions

Shaped single crystals of (Lu_xGd_1−x)₃Ga₅O₁₂ (0.0x1.0) and (Yb_0.05Lu_xGd_0.95−x)₃Ga₅O₁₂ (0.0x0.9) were grown by the modified micro-pulling-down method. Continuous solid solutions with garnet structure and a linear compositional dependency of crystal lattice parameter in the system Yb:(Gd,Lu)₃Ga₅O₁₂ are formed. Measured optical absorption spectra of the samples show 4f–4f transitions related to Gd³⁺ ion at 275 and 310 nm, and also an onset of charge transfer transitions from oxygen ligands to Gd³⁺ or Yb³⁺ cations below 240 nm. A complete absence of Yb³⁺ charge transfer luminescence under X-ray excitation in any of the investigated samples was explained by the overlapping of charge transfer absorption of Yb³⁺ by that of Gd³⁺ ions. For specific composition of Lu_1.5Gd_1.5Ga₅O₁₂ an intense defect-host lattice-related emission, which achieve of about 40% integrated intensity compared with Bi₄Ge₃O₁₂, was found.     

A new layered cuprate superconductor: GaSr2(Y, Ce)2Cu2O9−δ

A new layered cuprate compound with a nominal composition of GaSr₂Y_2−xCe_xCu₂O_9−δ has been prepared. It crystallizes in a tetragonal lattice with cell parameters: a = 3.812 Å, c = 28.16 Å. The structure of the compound belongs to the same family of 1222 phase and is derived from that of GaSr₂LnCu₂O₇ by replacing the single Ln³⁺ layer with a double fluorite (Y, Ce)₂O₂ layer. Like other parent cuprate compounds of superconductors, the as-prepared samples showed antiferromagnetic and semiconducting behavior. After treatment under high oxygen pressure, the samples exhibited bulk superconductivity with transition temperatures between 12–14 K.     

New red Y0.85Bi0.1Eu0.05V1−yMyO4 (M=Nb, P) phosphors for light-emitting diodes

The Y_0.85Bi_0.1Eu_0.05V_1−yM_yO₄ (M=Nb, P) as new near-ultraviolet excited phosphors were synthesized and their luminescence properties under 365 nm excitation were investigated in detail. It indicated that by doping small amount of P⁵⁺ into V⁵⁺ sites, the excitation intensity of charge transfer (CT) band of Bi–O (330–400 nm) was greatly improved. By substituting Nb⁵⁺ for V⁵⁺, both the CT bands of Bi–O and Eu–O (240–320 nm) were significantly enhanced. As a result, the emission intensity of Y_0.85Bi_0.1Eu_0.05V_1−yM_yO₄ (M=Nb, P) could be improved about 90% by doping 5 mol% P⁵⁺ and 110% by doping 5 mol% Nb⁵⁺. Comparing with the commercial Y₂O₂S:Eu³⁺ phosphors, the Y_0.85Bi_0.1Eu_0.05V_0.95M_0.05O₄ (M=Nb, P) phosphors exhibited excellent color purity and much higher brightness. The results showed that these Y_0.85Bi_0.1Eu_0.05V_1−yM_yO₄ (M=Nb, P) phosphors could be considered as promising red phosphors for application in LED.     

Characterization of nonstoichiometric nickel manganite spinels NixMn3−x□3δ/4O4+δ by temperature programmed reduction

Cation deficient spinels Ni_xMn_3−x□_3δ/4O_4+δ (0≤x≤1) have been prepared by thermal decomposition of mixed oxalates Ni_x/3Mn_(3−x)/3(C₂O₄)·nH₂O in air at 623 K. They have been characterised by temperature programmed reduction (TPR) under H₂, the reaction being followed by gravimetric and powder X-ray diffraction measurements. It has been shown that TPR proceeds in several steps. The first steps correspond to the loss of nonstoichiometric oxygen leading to the formation of a stoichiometric oxide. During the following stages the manganese cations are reduced, causing the spinel structure to be destroyed, and the formation of solid solution of NiO in a cubic MnO. Subsequently, Ni²⁺ cations undergo a reduction to metallic nickel, and, finally, a mixture of nonstoichiometric MnO_1−δ and metallic nickel is formed. These oxides contain a high level of vacancies which vary with the nickel content with a maximum of δ≈1 near x=0.6. This nonstoichiometry is ascribed both to the presence of Ni³⁺ and excess of Mn⁴⁺.     

Trapping mechanism of superconductivity suppression induced by Pr substitution in the Ho1−xPrxBa2Cu3O7−δ system

Pr concentration dependence of the superconducting transition temperature T_c in the Ho_1−xPr_xBa₂Cu₃O_7−δ system is determined from measurements of DC electrical resistance. This dependence coincides with that for the parallely studied Y_1−xPr_xBa₂Cu₃O_7−δ reference system. Both systems have the same value of the critical concentration x_c=0.58, in accordance with nearly equal ionic radii of Ho³⁺ and Y³⁺ ions. It has been shown that the T_c(x) curve can be described with a single mechanism based on a decreasing number of sheet holes trapped by Pr^IV-ions, if one takes also into account that the number of these ions changes with x.     

Al~(3+)/Mo~(6+)双离子取代ZrV_2O_7中Zr~(4+)/V~(5+)实现近零膨胀

采用固相烧结法制备了Zri_(1-x)Al_(2-x)V_(2-x)Mo_xO_7(0≤x≤0.9),并通过调整Al~(3+)/Mo~(6+)对ZrV_2O_7中的Zr~(4+)/V~(5+)离子替代量来实现近零膨胀,对于较小的x值(x≤0.3),材料保持了与ZrV_2O_7相同的立方相结构.随着Al~(3+)/Mo~(6+)替代量的增加,(Al/Zr)~-和(Mo/V)~+之间的库仑相互作用逐渐加强,这种库仑相互作用导致材料中未发生畸变的立方相晶体结构逐渐减少.当x≥0.7时,材料中立方相晶体结构完全消失.在425-750 K温度区间内,Zr_(0.5)Al_(0.5)M_(0.6)O_7展示出近零膨胀性质(-0.39×l0~(-6)K~(-1)).Zr_(0.5)Al_(1.5)V_(1.5)Mo_(0.5)O_7的低热膨胀性能可能与Al~(3+)/Mo~(6+)对ZrV_2O_7中Zr~(4+)/V~(5+)部分替代引起部分晶体结构发生的畸变及其对未替代部分的晶格结构的影响有关.     

Oxygen permeability of La2Cu(Co)O4+δ solid solutions

Formation of the La₂Cu_1−xCo_xO_4+δ solid solutions with orthorhombic K₂NiF₄-type structure was found to be in the range of 0≤x≤0.30 at temperatures above 1270 K. Incorporating cobalt into the copper sublattice of lanthanum cuprate leads to increasing oxygen hyperstoichiometry and decreasing electrical conductivity. Thermal expansion coefficients of the La₂Cu_1−xCo_xO_4+δ (x=0.02–0.30) ceramics at 470–1100 K were calculated from the dilatometric data to vary in the range (12.2–13.2)×10⁻⁶ K⁻¹. Studying the dependence of oxygen permeation fluxes through La₂Cu(Co)O_4+δ on the membrane thickness demonstrated that the oxygen transport at the thickness values below 1 mm is limited by both surface exchange rate and bulk ionic conductivity. Oxygen permeability of the La₂Cu_1−xCo_xO_4+δ solid solutions was ascertained to increase with cobalt concentration at x=0.02–0.10 and to decrease with further dopant additions, indicating a participation of interstitial oxygen in the ionic transport.     

Mössbauer investigations on Np3S5

The 59.5 keV Mössbauer resonance of ²³⁷Np has been measured in Np₃S₅ at 77 and 4.2 K. At 77 K, two different quadrupolar splittings are observed. The first one (δ₁ = −5(1) mm sec⁻¹/NpAl₂) is assigned to the Np⁴⁺ ions and the second one (δ₂ = 28(1) mm sec⁻¹/NpAl₂) is assigned to the Np³⁺ ions. At 4.2 K the spectrum gives a ratio of areas S(Np³⁺)/S(Np⁴⁺) of 2.0. These results clearly confirm the crystallographic data of this mixed-valence material.     

Realization and properties of ramp-type YBa2Cu3O7−δ/Au/Nb junctions

All-thin-film ramp type Josephson junctions between YBa₂Cu₃O_7−δ and Nb have been fabricated. This procedure allows connections between high-T_c and low-T_c superconductors at different crystal sides of the high-T_c superconductor on one chip, which is of great interest for novel phase devices. A thin Au layer is incorporated as a chemical barrier to avoid oxygen transfer from the YBa₂Cu₃O_7−δ to the Nb. Critical current densities up to 600 A/cm² are obtained at T=4.2 K, with typical R_nA values of 0.8 μΩ cm². The variation of the magnetic field dependence of the critical current with the angle between the junction barrier and the YBa₂Cu₃O_7−δ crystal axes is explained by considering a predominant d_x²−y² order parameter symmetry of the YBa₂Cu₃O_7−δ. The successful fabrication of these junctions allows the implementation of novel superconducting electronics, such as complementary Josephson circuitry or proposed qubit concepts, using the unconventional order parameter symmetry of the high-T_c superconductor.     

Fabrication and growth rate estimation of PrBa2Cu3O7−δ single crystal by the modified top seeded crystal pulling method

Recently, we succeeded in fabricating single crystals of PrBa₂Cu₃O_7−δ by a modified top seeded crystal pulling method called the SRL-CP (Solute Rich Liquid-Crystal Pulling) method. Y₂O₃ and MgO polycrystalline crucibles and a MgO single crystal crucible were used to grow the single crystals. The crystal growth temperature was set in the range of 968°C to 972°C. The grown crystals were identified as PrBa₂Cu₃O_7−δ by X-ray diffraction. In the case of using Y₂O₃ crucibles the composition of the grown crystals was Y_xPr_1−xBa₂Cu₃O_7−δ (0.48 < x < 0.57) and in the case of using MgO crucibles a relatively small amount of Mg contamination to the grown crystals occurred at a typical concentration of approximately 1 at.% of the sum of cations. According to the crystal growth model of the SRL-CP method [1–5], a maximum growth rate of 1.7 × 10⁻⁵ cm/s was calculated with the aid of the phase diagram studies we reported earlier [6]. This value is reasonably in agreement with the experimental results.     

Structural and electrical properties of crystalline (1−x)Ta2O5−xTiO2 thin films fabricated by metalorganic decomposition

Polycrystalline (1−x)Ta₂O₅−xTiO₂ thin films were formed on Si by metalorganic decomposition (MOD) and annealed at various temperatures. As-deposited films were in the amorphous state and were completely transformed to crystalline after annealing above 600 °C. During crystallization, a thin interfacial SiO₂ layer was formed at the (1−x)Ta₂O₅−xTiO₂/Si interface. Thin films with 0.92Ta₂O₅–0.08TiO₂ composition exhibited superior insulating properties. The measured dielectric constant and dissipation factor at 1 MHz were 9 and 0.015, respectively, for films annealed at 900 °C. The interface trap density was 2.5×10¹¹ cm⁻² eV⁻¹, and flatband voltage was −0.38 V. A charge storage density of 22.8 fC/μm² was obtained at an applied electric field of 3 MV/cm. The leakage current density was lower than 4×10⁻⁹ A/cm² up to an applied electric field of 6 MV/cm.     

Low-temperature AC conductivity of Bi2Sr2CaCu2O8+δ

We report measurements of anamolously large dissipative conductivities, σ₁, in Bi₂Sr₂CaCu₂O_8+δ at low temperatures. We have measured the complex conductivity of Bi₂Sr₂CaCu₂O_8+δ thin films at 100–600 GHz as a function of doping from the underdoped to the overdoped state. At low temperatures there exists a residual σ₁ which scales with the T=0 superfluid density as the doping is varied. This residual σ₁ is larger than the possible contribution to σ₁ from a thermal population of quasiparticles (QP) at the d-wave gap nodes.     

Infrared optical response of YBa2Cu3O7 and PrBa2Cu3O7 An ellipsometric study

Ellipsometry was used to study (110) and (001) oriented films of YBa₂Cu₃O₇ and PrBa₂Cu₃O₇ in the mid-and near-infrared spectral regions. Below a photon energy of 0.1 eV, the in-plane component of the dielectric tensor of YBa₂Cu₃O₇ is dominated by a Drude term with a squared plasma energy of (3.0 ± 0.3) eV². This “oscillator strength”, and the Lorentzian broadening energy of (0.104 ± 0.005) eV at room temperature, are confirmed by the changes induced in Pr-substituted material, and by low-temperature measurements in the near-infrared. The deviation from the Drude behavior observed above 0.1 eV is accounted for by a broad absorption band with an oscillator strength of (3.6 ± 0.1) eV² in YBa₂Cu₃O₇ which shifts to higher energies and takes over almost all of the oscillator strength of the Drude term when Y is substituted by Pr. The response to electric fields perpendicular to the planes is much weaker, with an upper bound of 0.63 eV² for the squared plasma energy. The in-plane loss function computed from the measured dielectric function follows the Drude-like lineshape, modified by the bound-state absorption band, down to the lowest energy reached in our measurements (0.058 eV).     

Dielectric behavior of hexaferrites BaCo2−xZnxFe16O27

The dielectric constant (′) and dielectric loss (tan δ) for hexaferrites BaCo_2−xZn_xFe₁₆O₂₇ have been studied as a function of frequency (f), temperature (T) and composition (x). The experimental results indicate that ′ and tan δ above the relaxation frequency only decrease as the frequency increases and as the temperature decreases. Tan δ shows the dielectric relaxation at certain critical frequencies which rise as temperature increases. The activation energy for the dielectric relaxation (E_D), ′, and tan δ are found to be minimum for x = 0.8.     

Synthesis and investigation of the complex copper oxides Bi2Sr2R2−xCexCu2O10+δ (R=rare earth)

Bi-2222 phases were prepared by solid state reactions and were studied using X-ray diffraction, chemical and EDS-analysis. Compounds with composition Bi₂Sr₂R_1.33Ce_0.67Cu₂O_10+δ were obtained for R=Pr, Nd, Sm-Er. The phases Bi₂Sr₂Cu₂O_10+δ with similar structure but without Ce were prepared for R = Pr, Sm-Dy. For Bi₂Sr₂Eu_2−xCe_xCu₂O_10+δ the solubility limit was determined and the effect of treatment under different oxygen pressures was investigated. Weak diamagnetic signals (<3%) were detected for some of these samples.     

Eu3+掺杂5Li2O-1Nb2O5-5TiO2发光陶瓷的制备及性能研究

以Li₂CO₃、Nb₂O₅、TiO₂和Eu₂O₃为原料,采用固相法制备Eu³⁺掺杂的5Li₂CO₃-1Nb₂O₅-5TiO₂（LNT）发光介质陶瓷。通过密度、XRD和荧光光谱测试,对0.2%（质量分数）Eu₂O₃掺杂的陶瓷片进行性能表征。结果表明：1 120℃烧结致密的陶瓷片,其晶相结构为“M-相”与Li₂TiO₃两相复合构成;在400 nm的近紫外光激发下,样品有较强的橙光（592 nm）和红光（615 nm）发射,分别属于Eu³⁺的⁵D₀→⁷F₁的磁偶极跃迁和⁵D₀→⁷F₂的电偶极跃迁。     

Study of the interfacial structure and chemistry of CVD κ-Al2O3/TiC multilayer coatings

This article describes the interfacial regions in CVD grown TiC/κ-Al₂O₃ multilayers. A number of microanalytical techniques were used including HREM, EDX and EELS. Occasionally, the first 50 nm of the alumina layers deposited on the intermediate TiC layers grew as a cubic alumina, heavily faulted, containing small amounts of sulphur (S), maybe as a stabiliser. The presence of slightly rounded TiC (111) facets may act as preferred nucleation sites for the cubic Al₂O₃ phase, with a ‘cube on cube’ orientation relationship. In this way the nucleation of κ-Al₂O₃ is less favourable. After some tens of nanometres the cubic phase cannot be stabilised any longer and the layer continues to grow as κ-Al₂O₃. A number of observations point towards the reaction zone (RZ) being η- and/or γ-Al₂O₃. The diffraction work and the FFT analysis of the HREM images show that the RZ is an fcc phase with a=7.9 Å, which matches with η- and γ-Al₂O₃. The EELS Al fine structure indicate more tetrahedral Al ions than in κ-Al₂O₃, as in η- and γ-Al₂O₃. The RZ contains small amounts of S, as has been reported for γ-Al₂O₃. Due to the structural similarities between η- and γ-Al₂O₃ it was not possible to determine which of these cubic phases is present in the RZ.     

Ho,Yb: Tb3Ga5O12纳米粉体制备及发光性能研究

以Tb₄O₇和Ga₂O₃(化学计量比为3: 5)、Ho₂O₃、Yb₂O₃为原料,其中Yb³⁺的掺杂浓度为8at.%,Ho³⁺的掺杂浓度分别为0.5at.%、1at.%、1.5at.%、2.0at.%,以碳酸氢铵为沉淀剂,在1 200 ℃下烧结10 h得到了Ho,Yb: Tb₃Ga₅O₁₂(Ho,Yb: TGG)纳米粉体。对样品进行了XRD物相分析、热重-差热分析、红外光谱分析以及扫描电镜分析、上转换发射光谱分析。实验结果表明,温度为1 200 ℃下样品平均晶粒尺寸为38.10 nm。在泵浦源为980 nm激发下,Ho³⁺掺杂浓度为1.5at.%,Yb³⁺掺杂浓度为8at.%时,在红、绿、蓝波段范围内出现了明显的上转换发光现象,并对其形成机理进行了讨论。分析认为,Ho³⁺由激发态⁵S₂,⁵F₄向基态⁵I₈跃迁,实现了绿光输出,而Ho³⁺由激发态⁵F₅和⁵F₃向基态⁵I₈跃迁,分别实现了红光和蓝光输出。