Alternate structure for Ba2YCu3O7

Based of known structural frameworks of oxygen-deficient perovskites, an alternate structure for Ba₂YCu₃O₇ is proposed. It is compatible with the X-ray data of that phase. The lattice is made up of perovskite layers of corner-sharing CuO₆ octahedra interlinked with corner-sharing square-planar CuO₄ units. The proposed structure makes the class of high-T _c superconductors with perovskite-related structures more consistent.     

Fe-site assignments in Y1Ba2Cu3O7 revisited

A new symmetric Fe³⁺ site (labelled E) (δ=0.28(2) mm/s and Δ=0.15 mm/s) is observed in the R₁Ba₂Cu₃O₇ family of superconductors for R=Y and Eu. The decay and growth of site E are established in two seemingly unrelated experiments, one as a function of room temperature aging for R=Y, and the other as a function of oxygen loading at elevated temperatures and pressures for R=Eu. In each experiment, the results show that site E is genetically related to the well documented chainsites A(Δ=1.9 mm/s), C(Δ=1.1 mm/s) and D(Δ=1.6 mm/s). We propose that site E represents a quasi-octahedral chain-site having pairs of O(1), O(4) and O(5) nearestneighbor oxygen sites. The two remaining doublets, B(Δ≈0.4 mm/s) and B′(Δ=0.78 mm/s) represent Fe³⁺ dopant sites present in the all-important CuO₂ planes, possessing coordination numbers of 6 and 5, respectively.     

NQR investigations of the high-Tc superconductor Ba2YCU3O7

NQR measurements of ^63,65Cu in both the normal and superconducting state are reported for the Cu1 and Cu2 sites in Ba₂YCu₃O₇. An anomalous behaviour in the vicinity and below T_c is observed: after a steep decrease at T_c the quadrupole frequency increases with decreasing temperature.     

Dielectric properties of Ba3Sr2DyTi3V7O30 ceramics

A polycrystalline sample, Ba₃Sr₂DyTi₃V₇O₃₀, with tungsten bronze structure was prepared by a mixed-oxide method at high temperature (950°C). Preliminary structural analysis of the compound showed an orthorhombic crystal structure at room temperature. Surface morphology of the compound was studied by scanning electron microscopy. The dielectric anomaly at 321°C may be attributed to the ferro-paraelectric phase transitions. This was also confirmed from the appearance of a hysteresis loop. The nature of variation of the ac conductivity and value of activation energy at different temperature regions, suggest that the conduction process is of mixed-type (i.e., ionic-polaronic and space charge generated from the oxygen ion vacancies).     

Photoluminescence Properties of Dy3+-Doped Ba2Be2B2O7

Journal of Applied Spectroscopy - Dy3+-doped Ba2Be2B2O7 was prepared and characterized using a combustion method and X-ray diffraction analysis. The photoluminescence (PL) properties of the...     

Electron tunneling into thin films of Y1Ba2Cu3O7

We have fabricated tunnel junctions on thin films of Y₁Ba₂Cu₃O₇ which were epitaxially grown by magnetron sputtering on (100) and on (110) oriented SrTiO₃ substrates. These junctions of the type Y₁Ba₂Cu₃O₇/barrier/Pb or In showed with high reproducibility in the conductance a gap-like structure with the maxima near±16mV. We supplied experimental arguments that this structure reflects properties of the quasi particle excitation spectrum of Y₁Ba₂Cu₃O₇. The gap-like structure was found to disappear atT _c mainly by weakening and not by a shift of the peaks to lower voltages.     

Kinetics of oxygen desorption in Y Ba2Cu3O7-δ

In this paper the kinetics of oxygen desorption from Y Ba₂Cu₃O_7-δ has been investigated and measurements carried out in the temperature range 350–750 C. It is shown that the oxygen desorption is diffusion-controlled (and not surface-controlled), and characteristic of diffusion from small grains in a loosely coupled porous matrix. At every temperature a number of time constants with well-defined ratios appear and have been identified and measured. These time constants can be related to the average bulk diffusion constant which shows an Arrhenius behaviour with deviations around 750°C which are ascribed to the well-known orthorhombic to tetragonal transition. In this paper we establish the basic underlying physical mechanism of oxygen desorption in Y Ba₂Cu₃O_7-δ.     

Ba2MgSi2O7:RE荧光粉发光性能的研究

采用高温固相法合成了Ba2MgSi2O7:RE(RE=Eu2 ,Ce3 ,Tb3 )系列荧光粉,研究了其结构和光谱特性.样品的XRD衍射图表明稀土离子的掺入没有引起基质晶格结构的变化.样品Ba2MgSi2O7:Ce3 在紫外光激发下呈蓝紫色发射,而样品Ba2MgSi2O7:Eu2 在紫外光激发下呈绿色发射.对样品Ba2MgSi2O7:Eu2 的光谱分析表明Eu2 在焦硅酸钡体系中可能占据两个不同格位.此外还分析了Ce3 ,Eu2 和Ce3 ,Tb3 共激活焦硅酸钡盐在紫外光激发下的光谱特性,并对其中存在的能量传递机理进行了讨论.     

Photoemission study of the Ba core levels in YBa2Cu3O7-x

Angle dependent X-ray photoemission spectra of epitaxial thin YBa₂Cu₃O_7-x films show characteristic differences after various heat and oxygen pretreatments. The component with the lowest binding energy in Ba 3d _5/2 and 4d _5/2 peaks results from bulk Ba in YBa₂Cu₃O_7-x . A component with 2 eV higher binding energy results from Ba in the surface unit cell with different bonding if compared with the bulk superconductor. This surface structure is observed for all YBa₂Cu₃O_7-x surfaces which have been exposed to oxygen. It is not observed for UHV prepared surfaces after mechanical scraping or fracturing.     

Metallic normal state of Y1Ba2Cu3O(7-delta)

Flux flow was studied over an entire temperature range down to T approximately 2% of T(c) by using intense pulsed current densities to overcome flux-vortex pinning. The resistivity at high vortex velocities is proportional to B and roughly follows rho approximately rho(n)B/H(c2), with a prefactor of order unity. Contrary to some speculation, rho(n) saturates to a finite residual value as T-->0, indicating a metallic (rho-->finite) rather than insulating (rho-->infinity) normal state, and the vortex dissipation continues to be conventional as T-->0.