Magnetic susceptibility and specific heat of Pr2?xCexCuO4?y

We report here the measurements of magnetic susceptibility of Pr_2–xCe_xCuO_4–y (x=0, 0.05 and 0.15) from 2K to 300K and the specific heat of Pr_1.85Ce_0.15CuO₄ between 6K and 60K. Fitting of the magnetic susceptibility data to a Curie-Weiss law gave effective magnetic moment (p_eff) values close to the free ion value of 3.58_B for Pr³⁺. p_eff was found to decrease with increase in cerium concentration. The site symmetry of Pr³⁺ in Pr_2–xCe_xCuO_4–y (PCCO) host is D_4h and a crystal field (CF) Hamiltonian appropriate to tetragonal symmetry was used to analyze the magnetic susceptibility data. J-mixing of all the thirteen multiplets of Pr³⁺ and intermediate coupled wave functions have been included. A best set of CF parameters obtained not only explains the magnetic susceptibility but also the inelastic neutron scattering spectra (INS) reasonably well. The Schottky component together with Debye and Einstein contributions successfully accounts for the measured specific heat.     

The Anomalous Optical Conductivity of Metallic La2?xSrxCuO4

The in-plane optical conductivity of three metallic La_2–x Sr _x CuO₄ single crystals with 0.10 x 0.15 has been studied between 30 and 295 K. Strong peaks in the far-infrared are observed, which cannot be explained by Drude-like models. Their similarity with peaks reported in Cu–O ladders with one-dimensional charge-ordering, their extremely low frequency, and their behavior with temperature allows us to conclude that those anomalous features are excitations of charge stripes in the Cu–O planes.     

Raman scattering investigation of the superconducting gap anisotropy in La2?xSrxCuO4

The low energy electronic Raman continua of a La_1.83Sr_0.17CuO₄ single crystal have been obtained in several different scattering geometries, and for temperatures above and below Tc. Based upon the polarization dependence of the spectra obtained for T Tc it is concluded that the superconducting gap is anisotropic with nodes located near the (±1,±1) directions in reciprocal space and maxima close to the kx and ky axes. The results are compared to spectra obtained from other high-Tc compounds and the possible compatibility of the results with particular gap functions is discussed.     

Stability of the low-temperature tetragonal phase in La2?xBaxCuO4 and related compounds

In La_2–x Ba _x CuO₄ (LBCO), the structural transition to a low-temperature tetragonal phase below 60 K and suppression of superconductivity are observed when the carrier density isp 1 /8 per copper. The replacement by divalent ions smaller than Ba²⁺ suppresses the static deformation of the lattice. We have found that the variationsT _d2 and superconducting transition temperatureT _c are quantitatively characterized by the averaged ionic radius at the La site or lattice parameters. This aspect of substitution could be regarded as the effect of chemical pressure, since similar variations have been reported on applying hydrostatic pressure. In La_2–x–y Nd _y (Ba, Sr) _x CuO₄,T _d2 increases with increasingy in a wide range ofp whileT _c is suppressed only at p l /8. The structural transition atT _d2 here should be ascribed mainly to the crystallochemical origin.     

The Optical Conductivity of La2?xSrxCuO4 in Connection with a Charge Stripe Scenario

The in-plane optical conductivity of La_2–xSr_xCuO₄ with 0.07 x 0.19 exhibits strong peaks in the far infrared. These features, which depend both on doping and temperature, can be associated with excitations of charge stripes, basing on results of different techniques on the same system and on recent theoretical calculations.     

Magnetic order and evolution of the electronic state aroundx=0.12 in La2?xBaxCuO4 and La2?xSrxCuO4

Muon spin rotation ( ⁺SR) measurement provides clear evidence of the antiferromagnetic order of Cu moments below 35 K for La_2–x Ba _x CuO₄ and below 15 K for La_2–x Sr _x CuO₄ in the narrow range ofx where the high-T _c superconductivity (SC) is suppressed remarkably. The results suggest that the change of the electronic state coupled with the lattice instability is relevant to the local suppression of SC and freezing of spin fluctuations of the Cu moment.     

On the high-temperature superconductivity of SrxLa2?xCuO4?δ

The superconducting properties of Sr_xLa_2–xCuO_4– are analyzed by employing the model of interaction between charge carriers and the oxygen-displacive modes of the lattice. The role played by the hole concentration, as related to the Sr content x and oxygen deficiency, is thoroughly emphasized. The critical temperature, the ratio of twice the superconducting gap to the critical temperature, and the isotope shift are estimated as functions of the hole concentration and compared with the experimental data.     

Synthesis and Structure of Hg1?xCrxSr2CuO4+δ Mercurocuprates

We report preliminary results concerning the synthesis and structural characterisation of the chromium stabilised 1201 phases: Hg_1–xCr_xSr₂CuO₄₊. A systematic study of the formation of phases in this system has been undertaken, together with a combined powder neutron and synchrotron x-ray diffraction study, to address the issue of chromium clustering and ordering in this system.     

On the Presence of Cu1+ in the Superconducting (Hg,M)Sr2CuO4+δ; M = Cr, Mo, or Re

The Cu K-edge measurements on the optimum T _c 1201 compounds (Hg_1–x M _x )Sr₂CuO₄₊ (M = Cr, Mo, or Re; 0.10 x 0.30) show besides Cu²⁺ (predominant) a very weak but distinct signature of Cu¹⁺, only in the superconducting (SC) samples. The Cu¹⁺ feature is conspicuously absent when these compositions are non-superconducting because of different processing conditions. Our finding of monovalent Cu in SC (Hg,M)/Sr-1201 favors the presence of Cu at the Hg-site and this seems to facilitate superconductivity in these cuprates.     

Room-temperature oxidation of La2?xSrxCuO4

A series of oxidized La_2–x Sr _x CuO_4+y compounds has been prepared by treating the starting materials with a solution of Br₂ in NaOH at room temperature. The structural modifications due to the oxidation of the materials have been studied by X-ray diffraction. Some of the observed changes are: (i) a large increase in the long parameter of the unit cell for samples with Sr content and (ii) a slight decrease in a along the whole range ofx. Interesting features have been observed regarding the critical temperatures of these materials: transition temperatures are higher for those containing lower Sr amounts (some of them were even nonsuperconducting before the oxidation treatment) in contrast to materials with x0.15, whoseT _c's do not change very much. The influence of both oxygen and strontium contents on the structural modifications and the superconducting properties of the oxidized materials will be discussed.     

M(M= Mg, Ca, Sr, Ba, Ni)Nb2O6的光催化性能的研究

应用固相烧结法制备了半导体光催化材料MgNb2O6,CaNb2O6,SrNb2O6,BaNb2O6,NiNb2O6粉体.利用XRD、UV-Vis漫反射谱、扫描电镜对上述粉体晶体结构、光谱性质和形貌进行了分析.在紫外光下对其降解罗丹明B（RhB）的光催化性能进行了评估.结果表明,碱土金属Sr、Ba对NiNb2O6中Ni的取代能很好地提高铌酸盐的光催化效果,在10h紫外光照射下能分别将RhB的浓度降解到原来的21%和37%.     

Experimental determination of the spatial distribution of electron defects in La2 ? xSrxCuO4 and Nd2 ? xCexCuO4 crystal lattices

The data of M?ssbauer emission spectroscopy on ⁶⁷Cu(⁶⁷Zn) and ⁶⁷Ga(⁶⁷Zn) isotopes show that holes appearing as a result of the Sr²⁺ substitution for La³⁺ in the La_{2 − x} Sr _x CuO₄ crystal lattice are localized predominantly at oxygen atoms occurring in the same atomic plane as the copper atoms. In contrast, electrons appearing as a result of the Ce⁴⁺ substitution for Nd³⁺ in the Nd_{2 − x} Ce _x CuO₄ crystal lattice are localized in the copper sublattice. These results are consistent with the model assuming that a mechanism responsible for the high-temperature superconductivity in La_{2 − x} Sr _x CuO₄ and Nd_{2 − x} Ce _x CuO₄ crystal lattices is based on the interaction of electrons with two-site two-electron centers possessing negative correlation energies (negative-U centers).     

Solvothermal synthesis of well-dispersed MF(2) (M = Ca,Sr,Ba) nanocrystals and their optical properties

MF(2)?(M = Ca,Sr,Ba) nanocrystals (NCs) were synthesized via a solvothermal process in the presence of oleic acid and characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectra, UV/vis absorption spectra, photoluminescence (PL) excitation and emission spectra, and lifetimes, respectively. In the synthetic process, oleic acid as a surfactant played a crucial role in confining the growth and solubility of the MF(2) NCs. The as-prepared CaF(2), SrF(2) and BaF(2) NCs present morphologies of truncated octahedron, cube and sheet in a narrow distribution, respectively. Possible growth mechanisms were proposed to explain these results. The as-prepared NCs are highly crystalline and can be well dispersed in cyclohexane to form stable and clear colloidal solutions, which demonstrate strong emission bands centred at 400?nm in photoluminescence (PL) spectra compared with the cyclohexane solvent. The PL properties of the colloidal solutions of the as-prepared NCs can be ascribed to the trap states of surface defects.     

Phase Separation in La2CuO4+δ by electrical transport measurements

The dc conductivity and the high-frequency capacitance of a La₂CuO₄₊ single crystal are determined by impedance spectroscopy in the frequency range 20 Hz to 1 MHz at liquid helium temperature. Using isochronal annealing, the effect of the phase separation on these quantitites is studied for E parallel and perpendicular to the CuO₂ planes as well as for different oxygen doping levels,, of the sample. The results are consistent with the model of a diffusion-controlled formation of a metallic—and belowT _c superconducting—percolation network in an insulating background.     

Ag掺杂对La2/3A1/3MnO3（A=Ca、Sr、Ba）材料电学性能的影响

采用化学共沉淀法制备了La2/3A1/3MnO3:Agx(A=Ca、Sr、Ba,LAMO:Agx,x=0、0.1、0.2、0.3、0.4 mol%)多晶复合材料。XRD结果表明实验制备了纯相LAMO:Agx样品,Ag+离子A位替代致使LCaMO:Agx和LSrMO:Agx晶格常数和晶胞体积发生微小膨胀,LBaMO:Agx晶格常数和晶胞体积发生微小收缩;R-T测试结果表明LCaMO:Agx的电阻温度系数(Temperature Coefficient of Resistance,TCR)数值在x=0.4、Tk=271.3 K(Tk为TCR位于最大值时对应的温度数值)时具有最大值为28%/K。SEM分析结果表明,随着Ag掺杂量增加,LCaMO晶粒尺寸增大,晶粒均匀性和规则性得以改善,晶化质量得到提高。通过Ag微观掺杂机制对LAMO复合材料的TCR数值增强进行了分析讨论。     

Hydrothermal synthesis of quasi-monodisperse AWO4 (A?=?Ca, Sr, and Ba) microspheres

In this study, quasi-monodisperse AWO₄ (A = Ca, Sr, and Ba) microspheres were fabricated by a facile hydrothermal route at 180 °C for 8 h in the presence of citric acid. The as-synthesized AWO₄ powders were characterized by X-ray powder diffraction (XRD), scanning electron microscopy, transmission electron microscopy and photoluminescence spectroscopy. The XRD results revealed that the hydrothermally formed AWO₄ powders presented a scheelite-type tetragonal structure, and the formation of a secondary phase was not observed. The SEM and TEM observations demonstrated that the AWO₄ powders with uniform sphere-like morphologies could be hydrothermally obtained at 180 °C for 8 h without the use of surfactants. The growth process of quasi-monodisperse BaWO₄ microspheres, as a representative of AWO₄, was investigated as a function of hydrothermal processing time, and a possible formation mechanism was proposed. The room temperature photoluminescence properties of AWO₄ powders were studied using an excitation wavelength of 350 nm. The positions of the PL emission peaks were not considerably altered, implying that the energy band gap relating to the blue emission was not much affected by the variation in chemical composition of AWO₄ (A = Ca, Sr, and Ba) microspheres. The obtained quasi-monodisperse AWO₄ microspheres will be potential candidates for a broad range of technological applications, as phosphors, luminescent materials, photocatalysts, etc.     

Synthesis, composition, and structure of superconducting (Y, Ca)Sr, Ba)2Cu2GaO7?δ

The superconducting properties of Y_1–y Ca _y Sr₂Cu₂GaO_7– have been examined and related to the Ca content,y, and the use of annealing treatments at 350 bar oxygen. Superconductivity withT _c up to 41 K was found only for high-pressure-annealed samples, and the structural effects of Ca substitution and high-pressure treatment were examined using powder neutron diffraction. Small but significant changes in Cu-Cu and Cu-O distances were found and suggest that the Cu ions are more highly charged in superconducting samples. Partial substitution of Ba for Sr was found to be possible (up to 20%) to give samples which, after annealing in high-pressure oxygen, were superconducting at temperatures up to 68 K.     

白光LED用红色荧光粉MMoO4:Eu3+（M=Ca,Sr,Ba）的制备与表征

采用高温固相法制备了红色荧光粉MMoO4:Eu3+(M=Ca,Sr,Ba),用XRD和荧光分光光度计对其物相及发光性能进行表征和研究。结果表明,在800℃时可得到MMoO4(M=Ca,Sr,Ba)物相结构。分别以395nm的近紫外光和465nm的可见光激发样品,MMoO4:Eu3+(M=Ca,Sr,Ba)荧光粉发红光,对应于Eu3+的4f-4f跃迁,其中以616nm发光最强。荧光粉在395nm和465nm的吸收分别与紫外光和蓝光LED芯片相匹配。     

The spin susceptibility of singlet correlated oxygen band in La2?xSrxCuO4

We have shown that the unconventional temperature dependence of the static susceptibility(T) of the perovskite high-T _c superconductors above the superconducting transition temperatureT _c can be explained in terms of two relevant band models containing the singlet-correlated oxygen band and the copper character band. The usual copper-oxygen Hamiltonian containing hopping and Coulomb repulsion terms has been reduced to an effective Hubbard-liket-t-t-U _eff model to describe the low-energy properties. The unusual behaviour of the susceptibility is due to thermally activated oxygen holes coming into the hybridization singularity peak in the density of states. A possible physical origin ofT _max in the temperature dependence of the susceptibility is discussed.     

Thermo-mechanical properties of La2NiO4+δ

Elastic and fracture behavior of La₂NiO_4+δ have been assessed. Fracture stress and elastic modulus of porous La₂NiO_4+δ were evaluated from room temperature (RT) up to 900 °C on the basis of 4-point bending tests. Both parameters increase slightly from RT to 700 °C. However, at higher temperatures the elastic modulus decreases, whereas the fracture stress increases. In addition, elastic modulus and damping/internal friction of dense specimens were measured by a resonance method. A strong change of elastic modulus and internal friction between RT and 100 °C appears to be related to an orthorhombic-tetragonal phase transition. No indications of phase transition can be observed at higher temperatures. Although thermogravimetric measurements suggest that oxygen was continuously released from the lattice up to 1000 °C with increasing temperature, the thermal expansion coefficient showed a rather stable value from RT up to 1000 °C.