Effect of mechanical activation on the phase composition,structure, and polymorphism of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O6 + δ oxide

The mechanical activation of YBa₂Cu₃O_6+δ powders is studied comprehensively using thermal analysis, scanning electron microscopy, high-temperature x-ray diffraction, and x-ray photoelectron spectroscopy. It is shown that the size of the coherent scattering regions is substantially reduced by intensive grinding (from 83 to 17 nm) and the material transforms to a nanostructured state. An increase in the reactivity of the material is found, which shows up as an enhanced accumulation of intercalated water in its structure. At the same time, no significant increase in the volume of impurity phases in the powder or in the influence of impurities on the crystal lattice parameters was observed. High-temperature x-ray studies of the lattice parameters of the YBa₂Cu₃O_6+δ phase reveal a jump (at temperatures of 430–630°C) and a decrease (above 630°C) in the parameter c when the powder is mechanically activated. A significant reduction in the temperature of the transition from the orthorhombic to the tetragonal structural modification is observed. These effects are explained by a change in the charge states of oxygen ions belonging to the base plane of the YBa₂Cu₃O_6+δ oxide during mechanical activation.     

退火诱导亚稳态Fe80Cu20合金固溶体的结构相变

利用扩展x射线吸收精细结构和x射线衍射研究了机械合金化制备的体心立方(bcc)的亚稳态Fe₈₀Cu₂₀合金固溶体的结构随退火温度的变化特点.结果表明，在300—873 K温度范围内，随着退火温度的升高，bcc结构物相的晶格常数近于线性降低，这主要是由于Cu原子从bcc结构Fe₈₀Cu₂₀合金固溶体中逐渐偏析出来，生成面心立方(fcc)结构的Cu物相所致.经603K退火后，Cu原子的平均键长R_Cu—Cu增加了0.003 nm左右，大约有50%的Cu原子从bcc结构的Fe₈₀Cu₂₀合金固溶体中偏析出来.在773 K退火后，bcc结构Fe_８０Cu₂₀合金固溶体近于完全相分离，生成了bcc结构的α-Fe与fcc结构的Cu物相. 关键词： 扩展x射线吸收精细结构 x射线衍射 ８０Cu₂₀合金')" href="#">Fe_８０Cu₂₀合金 机械合金化     

Synthesis and characterization of hollow spherical copper phosphide (Cu3P) nanopowders

In this paper, hollow spherical Cu₃P nanopowders were synthesized by using copper sulfate pentahydrate (CuSO₄?5H₂O) and yellow phosphorus in a mixed solvent of glycol, ethanol and water at 140–180 ^°C for 12 h. X-ray powder diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), electron diffraction pattern (ED) and transmission electronic microscopy (TEM) studies show that the as-synthesized nanocrystal is pure hexagonal phase Cu₃P with a hollow spherical morphology. Based on the TEM observations, a possible aggregation growth mechanism was proposed for the formation of Cu₃P hollow structures. Meanwhile, the effects of some key factors such as solvents, reaction temperature and reaction time on the final formation of the Cu₃P hollow structure were also discussed.     

Nanocrystalline Cu<Subscript>2</Subscript>O prepared under high pressures

The effect of high pressure on the structure of Cu₂O is investigated. Polycrystalline samples are treated during different periods of time at different temperatures and pressures in the vicinity of the kinetic curve of the decomposition Cu₂O → Cu+CuO. The structure of the samples subjected to thermobaric treatment is characterized by x-ray diffraction, electron diffraction structural analysis, and electron microscopy at atmospheric pressure. It is found that, upon treatment of Cu₂O at temperatures and pressures close to the decomposition region, the microstructure undergoes a transformation into the nanocrystalline state.     

Fabrication and characterization of Mn-doped Cu2O thin films grown by RF magnetron sputtering

Two series of Mn-doped Cu₂O diluted-magnetic-semiconductor thin films were prepared by radio-frequency (RF) magnetron sputtering. One is prepared at different deposition temperature with the same Mn doping concentration; the other is deposited at the same temperature but with varying Mn concentration. They were used to find out the ferromagnetic-order zone for the Mn-doped Cu₂O systems. Most of the samples show high (1 1 1) orientation, except low doping concentration (<6 at%). No impurities were found by X-ray diffraction and electron diffraction measurement. The doped Mn ions substituted Cu ions in the Cu₂O lattice and there were about 1.5% cation vacancies. The grains shown in the transmission electron microscopy (TEM) images for all the samples were tiny, i.e. just 5 nm in diameter. A rough phase diagram for the ferromagnetic order existing in the Mn-doped Cu₂O thin films was given with varying Mn doping concentration and deposition temperature.     

Investigation of phase transformations in Cu1.8S-Cu2S thin films by kinematic electron diffraction

The phase transformations of the compounds Cu_1.8S-Cu₂S have been investigated. All the changes in the sample due to a thermal treatment up to 300°C were continuously photographed by the kinematic electron diffraction method. It was shown that, depending on experiment conditions, the low-temperature ordered phase of digenite Cu_Z-XS transforms completely or partly into the hexagonal modification of chalcocite Cu₂S, which becomes ordered in its low-temperature orthorhombic modification. At temperatures from room to 100°C the orthorhombic chalcocite coexists with the low-temperature digenite, in a range from 100°C to 300°C the high-temperature hexagonal chalcocite coexists with the low-temperature digenite and at 300°C the sample contains a mixture of the hexagonal chalcocite and the f.c.c.-phase of digenite. Djurleite and other phases with intermediate compositions are not observed in these experiments.     

Spontaneous magnetostriction of Y2Fe16Al compound

The structure and magnetic properties of Y₂Fe₁₆Al compound have been investigated by means of x-ray diffraction and magnetization measurements. The Y₂Fe Fe₁₆Al compound has a hexagonal Th$_{2}$Ni$_{17}$-type structure. Negative thermal expansion was found in Y₂Fe₁₆Al compound in the temperature range from 332 to 438K by x-ray dilatometry. The coefficient of the average thermal expansion is \alpha =-3.4\times 10^-5K^-1. The spontaneous magnetostrictive deformations from 293 to 427K have been calculated based on the differences between the experimental values of the lattice parameters and the corresponding values extrapolated from the paramagnetic range. The result shows that the spontaneous volume magnetostrictive deformation \textit{$\omega $}$_{\rm S}$ decreases from 5.4$\times $10^-3to near zero with temperature increasing from 293 to 427K, the spontaneous linear magnetostrictive deformation \textit{$\lambda $}$_{\rm c}$ along the $c$ axis is much larger than the spontaneous linear magnetostrictive deformation \textit{$\lambda $}$_{\rm a}$ in basal-plane in the same temperature range except near 427K.     

Pressure-induced structural phase transition in rare-earth trihydrides. Part I. (GdH3, HoH3, LuH3)

High-pressure X-ray diffraction studies of gadolinum, holmium and lutetium trihydrides have been carried out in a diamond anvil cell up to 30 GPa at room temperature. A reversible structural phase transformation from the hexagonal to cubic phase has been observed for all the hydrides investigated. These results confirm our first discovery of the hexagonal to cubic phase transition in erbium trihydride published recently [T. Palasyuk, M. Tkacz, Solid State Commun. 130 (2004) 219. [1]]. The lattice parameters of the new cubic phases and the volume changes at transition points were determined for SmH₃, GdH₃, and HoH₃. The parameters of the equation of state for all the hexagonal and cubic phases of the investigated compounds have been determined.     

Structural phase transitions in Cu2–x Te crystals (x = 0.00, 0.10, 0.15, 0.20, 0.25)

Structural phase transitions in Cu_2–x Te crystals have been investigated by high-temperature X-ray diffraction.

At room temperature Cu₂Te and Cu_1.90Te specimens are two-phased, i.e. they consist of an orthohombic phase with a = 7.319, b = 22.236, c = 36.458 Å and a hexagonal phase with a = 4.150, c = 7.188 Å. The changes in both compounds take place generally in the hexagonal phase with increasing temperature. At 821, 873 K they transform to a FCC phase. Monocrystals of the other compounds at room temperature crystallize in the hexagonal system and at 673, 773, 723 K, respectively, they transform to a FCC phase. It is determined that as the cation deficiency increases the crystal quality becomes better.     

Spectral Investigations on Cu<Superscript>2+</Superscript>-Doped ZnO Nanopowders

Cu²⁺-doped ZnO nanopowders, synthesized at room temperature by mild and simple solution method, are characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), optical, electron paramagnetic resonance (EPR) and Fourier-transform infrared (FT-IR) techniques. From XRD and SEM, the crystal structure is identified as hexagonal, and the average crystallite size is around 53 nm. Lattice cell parameters are evaluated. The optical and EPR spectral investigations suggest that the Cu²⁺ ion enters the host lattice in two tetragonally distorted octahedral sites. Crystal field, tetragonal field, spin Hamiltonian and bonding parameters are estimated.     

Structural and magnetic properties of Li-Cu mixed spinel ferrites

Li_0.5−x/2Cu_xFe_2.5−x/2O₄ (where x=0.0-1.0) ferrites have been prepared by solid-state reaction. X-ray diffraction was used to study the structure of the above investigated ferrites at various sintering temperatures. Samples were sintered at 1000, 1100 and 1200 °C for 3 h in the atmosphere. For the sintering temperature of 1000 °C, Li_0.5−x/2Cu_xFe_2.5−x/2O₄ undergoes cubic to tetragonal transformation for higher Cu content. However, for the sintering temperature of 1100 and 1200 °C, X-ray diffraction patterns are mainly characterized by fcc structure, though presence of tetragonal distortion was found by other temperature dependence of initial permeability curves. The lattice parameter, X-ray density and bulk density were calculated for different compositions. Curie temperature was measured from the temperature dependence of initial permeability curves. Curie temperatures of Li-Cu mixed ferrites were found to decrease with the increase in Cu²⁺ content due to the reduction of A-B interaction. As mentioned earlier, temperature dependence of initial permeability curves was characterized by tetragonal deformation for the samples containing higher at% of Cu. The complex initial permeability has been studied for different samples. The B-H loops were measured at constant frequency, f=1200 Hz, at room temperature (298 K). Coercivity and hysteresis loss were estimated for different Cu contents.     

Low-temperature internal friction of modified Ti50Ni42Co8 alloy

In this work we have investigated the internal friction of the alloy Ti₅₀Ni₄₂Co₈ with Hf (0.5–0.8 at. %) and Y (0.1 at. %) additives in the temperature range 4.2–293 K at frequencies of 200–600 Hz. We see two peaks on the temperature dependence of the internal friction for the alloy oxidized in an air atmosphere at 1100 K: the first peak at T=40 K is connected with movement of interphase boundaries during the thermoelastic phase transformation occurring according to the scheme B2 R B19'; and the second peak at 220–240 K is mainly connected with diffusion under the pressure of hydrogen adsorbed on the oxide particles. The hydrogen penetrates the alloy during sample preparation upon etching and electropolishing in acid solution. Support for the latter hypothesis comes from internal friction vs temperature data for the alloy hydrogen-charged at 1073 K in a gaseous atmosphere and then subjected to vacuum annealing at the same temperature, as well as the results of x-ray diffraction analysis.Voronezh Polytechnical Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 14–20, December, 1992.     

Charge density waves in 2HNbSe2

The formation of a weak superlattice in transmission electron diffraction patterns of the hexagonal layered superconductor 2HNbSe₂ below 35 K is reported. Based on the temperature dependence of the superlattice reflexion intensity down to 17 K and by comparison with related layer materials, an interpretation of these effects in terms of a charge density wave coupled to a periodic lattice distortion is proposed.     

Electromagnetic response of a single phase Y-Ba-Cu-O compound superconducting in the 90 K range

The results of x-ray diffraction, electrical resistance, DC and AC magnetization studies on two specimens of compositions Y_2.1Ba_3.9Cu₆O_14−δ and Y_1.8Ba_4.2Cu₆O_14−δ are presented. Our studies confirm that La₃Ba₃Cu₆O₁₄ type orthorhombic phase is responsible for superconductivity in the 90 K range. The heat treatment that yields high quality samples with sharp transition in electrical resistance as well as in AC magnetic susceptibility measurements is described. Magnetic response just belowT _c is found to be sensitive to the measuring field.     

Lack of oxygen substitution in the chevrel compound Mo6S8

We present results from x-ray powder diffraction and lithium intercalation cells on samples of nominal composition “Cu₂Mo₆S₆O₂” and “Mo₆S₆O₂”. Our results show that it is not possible to prepare these materials with any significant fraction of the sulfur substituted by oxygen as claimed by Umarji et al.¹ Instead, the high temperature (1250°C) products from starting materials of CuO, Cu, S, MoO₂ and/or Mo are MoO₂, Mo and Cu_yMo₆S₈ with y>2.     

Influence of (Mn or Co)-doping on structural,magnetic and electronic properties of nano Zn0.75Cd0.25S

Nano Zn_0.75Cd_0.25S doped with Mn or Co was prepared using the thermolysis method. The nano nature and particle morphology of the formed samples were investigated by the transmission electron microscope. The effect of doping with Co or Mn on the structural characteristics was investigated. Also, the phase percentage of cubic zinc-blende and hexagonal wurtzite phases, developed in the prepared samples, was characterized in detail using Rietveld analysis for synchrotron x-ray diffraction data. An extra phase (CoS) is formed in the sample doped with 20% Co, while the sample with 20% Mn- doping has no extra phase. The behavior of lattice parameters of zinc-blende and wurtzite phases upon doping and the occupancies of magnetic ions were also examined. Magnetic measurements revealed the transformation of Zn_0.75Cd_0.25S from diamagnetic nature to a weak ferromagnetic upon doping it by either 20% Co or 20% Mn. Density function theory calculation was used to understand the ferromagnetic behavior of the doped samples and also to investigate the effect of doping on the other optical parameters.     

Anisotropy of the vortex structure and resistivity in the basal plane of YBa<Subscript>2</Subscript>Cu<Subscript>4</Subscript>O<Subscript>8</Subscript> single crystals

The vortex lattice of the YBa₂Cu₄O₈ high-temperature superconductor is studied in the basal plane of monocrystalline samples using the decoration technique in a field interval of 40–600 Oe. Vortex lattice anisotropy (field-independent “compression” of a regular hexagonal vortex cell in the poorly conducting direction by a factor of about 1.3) is detected. Resistivity anisotropy ρ _a /ρ _b measured at temperatures from T _c to room temperature is 16–9. Possible reasons for the discrepancy between our results and the available data are discussed.     

Phase transformation of strontium hexagonal ferrite

The phase transformation of strontium hexagonal ferrite (SrFe₁₂O₁₉) to magnetite (Fe₃O₄) as main phase and strontium carbonate (SrCO₃) as secondary phase is reported here. SrFe₁₂O₁₉ powder was obtained by a heat treatment at 250 °C under controlled oxygen flow. It was observed that the phase transformation occurred when the SrFe₁₂O₁₉ ferrite was heated up to 625 °C in confinement conditions. This transformation took place by a combination of three factors: the presence of stresses in the crystal lattice of SrFe₁₂O₁₉ due to a low synthesis temperature, the reduction of Fe³⁺ to Fe²⁺ during the heating up to 625 °C, and the similarity of the coordination spheres of the iron atoms present in the S-block of SrFe₁₂O₁₉ and Fe₃O₄. X-ray diffraction analysis confirmed the existence of strain and crystal deformation in SrFe₁₂O₁₉ and the absence of them in the material after the phase transformation. Dispersive X-ray absorption spectroscopy and Fe⁵⁷ Mössbauer spectroscopy provided evidences of the reduction of Fe³⁺ to Fe²⁺ in the SrFe₁₂O₁₉ crystal.     

Temperature dependence of oxygen content and phase transition in Y1Ba2Cu3O9−x compounds

Systematic TGA (Thermogravimetric analysis), magnetic susceptibility measurements and X-ray diffraction investigations have been made on the temperature dependence of oxygen content and phase transition in Y₁Ba₂Cu₃O_9–x compounds. The experimental results show that reversible oxygen relief and absorption appeared during heating and cooling in the temperature range of 400–950°C. A low cooling rate leads to higher oxygen content and quenching can suppress oxygen absorption. The deoxidation process is accompanied by a change of the cell constants,a andb increase whilec decreases with reduced oxygen content while the phase transition from the ortorhombic to the tetragonal system appeares. Nevertheless, this transition is a gradually changing process, no critical point of transition exists. Quenching stress and defects have a pronounced influence on the absorption and relief of oxygen. Maximum rates of oxygen relief and absorption were obtained at about 650°C. Oxygen absorption and desorption can be obtained only in an athermal process. Magnetic susceptibility measurements show that diamagnetic and paramagnetic suceptibilities increase obviously with increasing oxygen content. Deoxidation and oxygen absorption can be restrained by adding Fe to the Y₁Ba₂Cu₃O_9–x phase, which suggests that deoxidation and oxygen absorption are very sensitive to crystal structure and lattice distortions.     

Existence of an incommensurate phase in a Li2B4O7 crystal

The x-ray diffraction spectra of Li₂B₄O₇ single crystals are investigated in the temperature range 80–300 K, and the lattice parameter c is determined in the same temperature range in the presence of a periodically varying temperature field. An incommensurate phase is not observed anywhere in the temperature range investigated, regardless of whether the crystals are subjected to a periodic temperature field. Fiz. Tverd. Tela (St. Petersburg) 39, 1461–1463 (August 1997)