Superconductivity, Ca content and oxygen deficiency of Ga(Sr,Ca)2(Yb,Ca)Cu2O7

Superconducting transition temperature (T_c), Ca content and oxygen deficiency are studied on GaSr_1.8Ca_0.2Yb_1−xCa_xCu₂O₇ (x≤0.35). Superconducting samples with T_c=52 K are prepared after the annealing at 20 MPa of oxygen. The T_c is reduced through a slight oxygen loss accompanied by annealing in air above 650°C. The oxygen loss suggests the presence of short Cu–O chains in the GaO₄ slab. The formal valence of planar Cu required for the appearance of superconductivity depends on oxygen and Ca contents. The critical formal Cu valences are 2.105 and 2.125 for the samples annealed in air at 600°C and at 835°C, respectively. The values are higher than those of usual high-T_c superconductors. This can be explained by a high concentration of localized holes in the CuO₅ slab.     

Electron paramagnetic resonance measurements for (La,Y)2/3(Ca,Ba)1/3MnO3

We have measured the paramagnetic (PM) resonance behavior as a function of temperature for various manganese perovskites (La,Y)_2/3(Ca,Ba)_1/3MnO₃ with an average A-site size r_A basically covering the whole region of ferromagnetic (FM) metallic ground state. We show that at least three regions with different magnetic behaviors can be distinguished by two phase lines: T_onset−r_A and T_c−r_A. For each given r_A, the complete PM and long-range FM behaviors appear above T_onset and below T_c, respectively; while some anomalous PM behaviors appear for the intermediate temperature range of T_onset<T<T_c . A possible magnetic transition process is discussed in order of decreasing temperature from high-T complete paramagnetism to low-T long-range ferromagnetism.     

Factors affecting coercivity in (Y,Sm,Tm)3(FeGa)5O12 and (Y,Sm,Lu,Ca)3(FeGe)5O12 LPE films

Factors which influence coercivity, H_c, in (Y,Sm,Lu,Ca)₃(FeGe)₅O₁₂ films and in (Y,Sm,Tm)₃(FeGa)₅O₁₂ films grown by LPE have been identified. An anomalous layer at the film-substrate interface exhibits coercivity values different from that of the middle, bulk, portion of the film. The contribution of the transient layer at the substrate interface could be reduced by increasing the rotation rate while immersing the substrate into the melt. Films containing Ga show lower coercivities than films containing CaGe possibly because films with Ga are more uniform in composition. Films with Ga show increased coercivities with increasing growth rates and with increasing Sm content throughout the film.     

Phase transition and superconductivity of (Ba,Sr,La)2YCu3Oz (z>6.9)

A series of (Ba,Sr,La)₂YCu₃O_z (z>6.9) samples is studied to elucidate the relationship between structure, cation composition and oxygen content. The concentration of La plays an important role in the orthorhombic to tetragonal phase transition. The tetragonal structure is stabilized by increasing oxygen content and decreasing average ionic radius of cations on the Ba site. The phase transition is explained in terms of the random distribution of CuO₅ pyramids in the BaO–CuO_x slab. Superconductivity does not depend significantly on the phase transition.     

Photoluminescence properties of Sr(Y, Gd)2O4:Euunder VUV excitation

Vacuum ultraviolet (VUV) excitation and photoluminescence (PL) properties of Sr(Y, Gd)₂O₄ doped with Eu³⁺ were studied. The excitation spectra of SrY_1.9Eu_0.1O₄ and SrY_1.0Gd_0.9Eu_0.1O₄ had absorption in the VUV region with the absorption band edge at 149 nm, while the absorption of SrGd_1.9Eu_0.1O₄ in the VUV region was weak, which could be due to the narrow host band gap and no efficient energy transfer occurred in the VUV region. The PL spectra of all samples exhibited the characteristic emission of Eu³⁺ with the red ⁵D₀-⁷F₂ transition (611 nm) being the most prominent group.     

Luminescence properties of Eu2+ in M2MgSi2O7 (M=Ca, Sr, and Ba) phosphors

The photoluminescence properties of alkali-earth magnesium silicates (M₂MgSi₂O₇, M=Ca, Sr, and Ba) doped with Eu²⁺ were investigated. Solid solutions of Ba _x Sr_2−x Si₂O₇, Ca₂MgSi₂O₇, and Sr₂MgSi₂O₇ were prepared. Ba _x Sr_2−x Si₂O₇ retained a tetragonal crystal structure similar to the structure of the other compounds up to a stoichiometry of x=1.6, which enabled a systematic study of the common structure. Monoclinic Ba₂MgSi₂O₇ was prepared, and the luminescence properties were compared with those of other samples. The emission and excitation spectra of tetragonal M₂MgSi₂O₇ (M=Ca, Sr, and Ba) changed as a function of the covalency, site symmetry, and crystal field strength. The luminescence properties showed excellent agreement with theoretical predictions based on these factors. The Stokes shift differentiated the emission behaviors of the tetragonal and monoclinic structures.     

YBa2(Cu0.95M0.05)3O7-δ的代换效应及其中子衍射研究

X射线衍射实验表明YBa₂(Cu_0.95M_0.05)₃O_7-δ(M=Ti,V,Cr,Mn,Fe,Co,Ni,Cu和Zn)均为单相结构。Fe,Co,Ni和Zn对Cu的替代使超导临界温度T_c显著下降,而同样含量的Ti,V,Cr,Mn对Cu的替代并未对超导性能产生显著影响。并利用中子衍射分析了Ti,Mn,Fe和Co对Cu原子的取代,发现代换原子对Cu的两个晶位各自存在不同的择优占据 关键词：     

Synthesis and photoluminescence characteristics of (Sr, Ca)3B2O6:Eu for application in white light-emitting diodes

A series of yellow-green (Sr, Ca)₃B₂O₆:Eu phosphors have been synthesized using precursors prepared via a facile sol-gel route. The solid-solution phases crystallized to materials with the formula of Sr_3−x−yCa_xEu_yB₂O₆ with varied Ca²⁺ and Eu²⁺ contents. The emission peak centered at 540 nm under near-UV excitation exhibited a broad-band distribution in the range of 450-650 nm. The dependences of the luminescence intensity on the contents of Ca²⁺ substitution and Eu²⁺ dopant were also investigated. The composition in the host lattice sensitively affected the chromaticity index. Sr_1.21Ca_1.7Eu_0.09B₂O₆ (SCB:0.09Eu) was shown to possess the highest intensity and broadest emission band. Calcining temperature was shown to greatly influence the luminescent properties of SCB:0.09Eu. It is concluded that SCB:0.09Eu can be used as an efficient yellow-green phosphor for white light-emitting diodes (white LEDs) applications.     

Antiferromagnetic transition in botallackite Cu2Cl(OH)3

The natural cuprate botallackite, Cu₂Cl(OH)₃, is found to be a new antiferromagnet with Magnetic susceptibility properties under strong field show non-linear M-H properties indicating metamagnetism. The T_N and the super-exchange coupling are discussed and compared with its polymorph atacamite and other copper oxides on the basis of their structural parameters.     

Electronic structures and ferroelectric properties of ABi2Ta2O9 (A=Ca, Sr, and Ba)

The electronic structures of ABi₂Ta₂O₉ (A=Ca, Sr, and Ba) were calculated by using first-principles under optimized structure. As the size of A-site cation decreases from that of Ba²⁺ to Ca²⁺, the band-gap between O 2p and Ta 5d increases from 2.0 to 2.9 eV, which responses to the stronger orbital hybridizations between Ta 5d and O 2p orbits favoring improvement of the ferroelectric property, decrease in leakage current, and increase in both spontaneous polarization and Curie temperature by the structural distortion. In contrast to CaBi₂Ta₂O₉ and SrBi₂Ta₂O₉, the hybridization between Ba 5p orbits and O 2p orbits in BaBi₂Ta₂O₉ has better structural stability.     

Preparation-dependent properties of Ca(Cu,Mn)7O12 CMR materials

Both magnetic ordering temperatures and lattice constants were observed to be a function of preparation temperature for single-phase CaCuMn₆O₁₂ CMR ceramics of the same cation, oxygen composition and crystal structure. It was found for the first time that higher preparation temperatures result in a linear increase of a cubic cell parameter a, , and in a sharp decrease of magnetic ordering temperatures , by about , for quenched samples. High resolution NPD experiments have shown the presence of antisite defects resulting in partial occupation of octahedral (8c) sites by Cu²⁺ at higher preparation temperatures as never assumed for an ideal structure. The data obtained give a new insight to the problem of functional properties reproducibility suggesting stronger requirements for the production control of CMR materials.     

Preparation and luminescent properties of phosphor MGd2(MoO4)4: Eu (M=Ca, Sr and Ba)

Intense red emitting phosphors MGd₂(MoO₄)₄: Eu³⁺ (M=Ca, Sr and Ba) have been synthesized by the simple sol-gel technique. The formation processes and the phase impurity of phosphors are characterized by thermogravimetry-differential thermal analysis (TG-DTA) and power X-ray diffraction (XRD). The narrower size distribution and the regular shape of the phosphor particles are also measured by Field emission scanning electronic microscopy (FE-SEM). Photo-luminescent properties of the phosphors are performed at room temperature. Their excitation spectra present strong absorption at 395 nm near-UV light and 465 nm blue light, which match well with commercial LED chips. The phosphors exhibit satisfactory and excellent red light dominated by 616 nm and their photoluminescence intensity is about 3-4 times stronger than that of phosphor YAG under the 465 nm excitation. In addition, the optimal concentrations of Eu³⁺ for phosphors MGd₂(MoO₄)₄ (M=Ca, Sr and Ba) have also been determined.     

Electronic structure of (Nd, Ce)2(Ba, Nd)2Cu3Oy

The dependence of T_c on oxygen content was determined for (Nd_0.67Ce_0.33)₂(Ba_0.67Nd_0.33)₂Cu_3.01O_y. It was found that superconduction appears in a range of y=9.10−9.16 and that T_c is enhanced remarkably by the increase of oxygen content. In an oxygen content range where superconduction appeared, the Ce valence was nearly +4 and the Cu valence changed from 2.31 to 2.33. The conductivity was measured in a wide temperature range between T_c and 1200 K as a function of oxygen partial pressure. The high-temperature conductivity increased with increasing oxygen content. It was found that both carrier concentration and mobility are increased with oxygen content. The conduction at low temperatures was semiconductor-like, and increased with increasing oxygen content, where the energy gap E_A-E_V between acceptor level and top of filled band approached zero.     

Magnetic disaccommodation in Sr hexagonal ferrites with X-phase (2SrO·15Fe2O3) initial composition

The relaxation of the initial permeability has been measured in polycrystalline Sr hexaferrites with the initial composition of X-phase (2SrO·15Fe₂O₃) in the temperature range between 80 and 500 K. The time decay of the initial permeability after sample demagnetization is represented by means of isochronal disaccommodation curves which show the presence of different disaccommodation processes whose maxima lie at 380, 300 and 160 K (resp. A, B and D peaks). This behaviour is explained regarding the spectra corresponding to barium ferrites in order to ascribe the different relaxation processes found for ionic transitions in the cationic sites within the hexagonal structure.     

Synthesis of (Ca,Ce,Ce)2Ti2O7: a pyrochlore with mixed-valence cerium

Pyrochlore with mixed-valence Ce was synthesized by firing and annealing Ce(NO₃)₄, TiO₂, and Ca(OH)₂ with a stoichiometry of CaCeTi₂O₇ at 1300 °C. The product contains Ce-pyrochlore, Ce-rich perovskite, CeO₂ (cerianite), and minor CaO. Electron energy-loss spectroscopy (EELS) revealed both Ce⁴⁺ and Ce³⁺ in the Ce-pyrochlore with a Ce⁴⁺ to total Ce (Ce⁴⁺/ΣCe) of 0.80 giving . Cerium in the perovskite and cerianite is dominated by Ce³⁺ and Ce⁴⁺, respectively. High-resolution transmission electron microscope (HRTEM) images show that the boundary between Ce-pyrochlore and Ce-rich perovskite is semi-coherently bonded. The orientational relationship between the neighboring Ce-pyrochlore and Ce-rich perovskite is not random. Ce-pyrochlore (CaCeTi₂O₇) is a chemical analogue for CaPuTi₂O₇, which is a proposed ceramic waste form for deposition of excess weapons-usable Pu in geological repositories. It is postulated, based on the presence of Ce³⁺ in the Ce-pyrochlore, that neutron poisons such as Gd can be incorporated into the CaPuTi₂O₇ phase.     

HRTEM study of new superconducting lead cuprate (Pb, Cu) (Eu, Ce)2(Eu, Sr)2Cu2Oz

Crystallographic properties of a new lead cuprate superconducting material (Pb, Cu) (Eu, Ce)₂(Eu, Sr)₂Cu₂O_z are examined by electron probe micro-analysis and transmission electron microscopy. It is confirmed that although the sample examined here is not of single-phase, the composition of the superconducting phase is close to the nominal composition. From TEM observations, it follows that the crystal structure of the new phase is similar to that of Tl-1222 which consists of a fluorite type block, a rock salt type layer and a perovskite block.     

Double exchange model for RuSr2(Eu,Gd)Cu2O8

We propose a double exchange model to describe the RuO₂ planes of RuSr₂(Eu,Gd)Cu₂O₈. The Ru⁺⁵ ions are described by localized spins, and additional electrons provided by the superconducting CuO₂ planes are coupled ferromagnetically to them by Hund rules coupling. We calculate the spin structure factor, magnetic susceptibility and magnetization as a function of magnetic field and temperature, using a Monte Carlo algorithm in which the Ru⁺⁵ spins are treated as classical. Several experiments which seemed in contradiction with one another are explained by the theory.     

Optical properties of (ZnO)0.5(P2O5)0.5 glasses doped with Gd2O3:Eu nanoparticles and Eu2O3

Binary (ZnO)_0.5(P₂O₅)_0.5 glasses doped with Eu₂O₃ and nanoparticles of Gd₂O₃:Eu were prepared by conventional melt-quench method and their luminescence properties were compared. Undoped (ZnO)_0.5(P₂O₅)_0.5 glass is characterized by a luminescent defect centre (similar to L-centre present in Na₂O-SiO₂ glasses) with emission around 324 nm and having an excited state lifetime of 18 ns. Such defect centres can transfer the energy to Eu³⁺ ions leading to improved Eu³⁺ luminescence from such glasses. Based on the decay curves corresponding to the ⁵D₀ level of Eu³⁺ ions in both Gd₂O₃:Eu nanoparticles incorporated as well as Eu₂O₃ incorporated glasses, a significant clustering of Eu³⁺ ions taking place with the latter sample is confirmed. From the lifetime studies of the excited state of L-centre emission from (ZnO)_0.5(P₂O₅)_0.5 glass doped with Gd₂O₃:Eu nanoparticles, it is established that there exists weak energy transfer from L-centres to Eu³⁺ ions. Poor energy transfer from the defect centres to Eu³⁺ ions in Gd₂O₃:Eu nanoparticles doped (ZnO)_0.5(P₂O₅)_0.5 glass has been attributed to effective shielding of Eu³⁺ ions from the luminescence centre by Gd-O-P type of linkages, leading to an increased distance between luminescent centre and Eu³⁺ ions.     

Unusual luminescence of octahedrally coordinated divalent europium ion in Cs2MP2O7 (M=Ca, Sr)

The excitation and emission spectra of octahedrally coordinated europium ion (Eu²⁺) ions in Cs₂M²⁺P₂O₇ (M²⁺=Ca, Sr) are reported and discussed. The remarkable features of the Eu²⁺ luminescence in these phosphate materials include (a) very large Stokes shift of emission (∼1 eV), (b) high luminescence quenching temperature, and (c) unusually low energy of the emitted photons for Eu²⁺ luminescence in phosphate-based materials. The broad emission bands of Eu²⁺ in Cs₂CaP₂O₇ and Cs₂SrP₂O₇ peak at 607 and 563 nm, respectively. The Stokes shift, crystal field splitting, centroid shift and the red shift of the Eu²⁺ 4f⁶5d¹ electronic configuration have been estimated from the relevant optical data. The radiative lifetime of the Eu²⁺ emission in Cs₂M²⁺P₂O₇ is ∼1.2 μs. The nature of the Eu²⁺ emission in Cs₂M²⁺P₂O₇ is discussed and arguments are presented to associate the luminescence with an extreme case of normal 4f⁶5d¹→4f⁷[⁸S_7/2] emission.     

Magnetic susceptibility studies of (CH2)3(NH3)2FeCl2Br2 and (CH2)6(NH3)2FeCl2Br2

The magnetic susceptibility of the layered compounds (CH₂)₃(NH₃)₂FeCl₂Br₂ and (CH₂)₆(NH₃)₂FeCl₂Br₂ has been measured in the range 80 < T < 300 K. The results follow a Curie-Weiss behavior in the range 120 < T < 300 K but are field dependent for T < 120 K. The results are interpreted in terms of a two-dimensional antiferromagnetic interaction which is canted. A comparison with the corresponding pure chloride compounds is given.