The superconducting and the electrical properties of single crystals Bi2Sr2CuCa2oXat high pressure

Abstract

We have investigated the effect of pressure on lattice parameters and Raman-active phonons of the molecular crystal xenon difluride by powder x-ray diffraction and first-order Raman scattering, respectively. The ambient pressure tetragonal crystal structure remains stable up to the maximum pressure of 50 GPa. The compressibility is found to be strongly anisotropic. The frequencies of the intramolecular A1_g stretching mode and the E_g libration increase sublinearly under pressure. ding the a-and c-axis compression data, we have determined the dependence of phonon frequencies on lattice parameters. The high-frequency A1_g mode shows a pronounced nonlinear scaling with c-axis lattice parameter, with particularly small changes during initial compression. This effect is attributed to the interplay between intra-and intermolecular interactions during compression     

High resolution x-ray diffractometry of the structural characteristics of a semiconducting (InGa)As/GaAs superlattice

A statistical approach is used to construct a kinematic theory of x-ray diffraction on a semiconducting superlattice with a two layer period. This theory takes two types of structural deformations into account: crystal lattice defects caused by microdefects distributed chaotically over the thickness of the superlattice, and periodicity defects of an additional superlattice potential owing to random deviations in the thicknesses of the layers of its period from specified values. Numerical simulation is used to illustrate the effect of structural defects on the development of the diffraction reflection curve. The theory is used to analyze experimental x-ray diffraction spectra of semiconducting In_xGa_1−x As/GaAs superlattices. Zh. Tekh. Fiz. 69, 44–53 (February 1999)     

Scintillation characteristics of thin-film calcium iodide crystal x-ray detectors

A container design for fabricating low-energy x-ray scintillation detectors is proposed. CaI₂ and CaI₂:Eu crystal detectors are fabricated and their characteristics are investigated. It is shown that on account of their layered structure, perfect cleavage, and high light output, calcium iodide scintillators can be used to fabricate thin-film detectors for long-wavelength x rays. Zh. Tekh. Fiz. 68, 87–90 (August 1998)     

Influence of crystal dimensions on interatomic distances in dispersed carbon

The relationship between the interatomic distances and crystal dimensions in dispersed carbon is studied by x-ray structure analysis and by performing model calculations. It is established that the interatomic distances in dispersed carbon are determined by the dimensions of the crystals along the crystallographic a axis (L _a). Small crystal dimensions dictate smaller interatomic distances than in graphite; an increase in the crystal dimensions leads to a corresponding increase in this parameter. The interatomic distances in dispersed carbon depend on the degree in covalency of the bonding, which is a function of L _a. Fiz. Tverd. Tela (St. Petersburg) 41, 744–747 (April 1999)     

Method of phase-dispersion introscopy

A method is described for studying the internal structure of noncrystalline objects which are weak x-ray absorbers. The method uses the conventional arrangement of plane-wave topography. The object to be investigated is immersed in a monochromatic pseudoplane wave formed by a highly asymmetric reflection off a perfect crystal or several crystals and introduces phase disruptions at the wave front. A perfect single crystal mounted behind the object in a Laue arrangement reveals these disturbances, forming a contrast image of the boundaries of the structural components of the object. The possibilities of the method are illustrated by photographs of objects of various natures. The features of the image contrasts are discussed and confirmed by experiments on test objects. Zh. Tekh. Fiz. 67, 68–77 (January 1997)     

Anomalous behavior of the structural parameters of YBa2Cu4O8 single crystals at the transition to the superconducting state

High-precision x-ray crystallographic studies of YBa₂Cu₄O₈ single crystals (T _c =70 K) are performed at eight temperatures in the interval 20–295 K. It is found that a number of structural parameters exhibit anomalous behavior near the superconducting transition of the crystal. A characteristic effect near the phase transition is the displacement of the O1 bridge atom that joins the Cu1 atom of the cuprate chain to the Cu2 atom of the cuprate plane. The shift of this oxygen toward the Cu2 atom is indicative of a change in the Cu2-O1 chemical bond and of charge transfer to the cuprate plane in the process of the transition of the crystal to the superconducting state. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 7, 502–506 (10 October 1997)     

Sulfide passivation of a textured interface of a gallium arsenide surface-barrier photovoltaic cell

A comparative study is made of the influence of sulfide passivation in an aqueous Na₂S·9H₂O solution on the photoconversion parameters of solar radiation in Au-GaAs barrier structures as a function of the character of the microrelief and the pretreatment of the GaAs surface. A quasigrating and a dendritic surface microrelief were produced by anisotropic chemical etching. It is shown that this type of GaAs surface treatment is potentially useful for enhancing the efficiency of a photovoltaic cell stored for several years. A possible mechanism is discussed for the processes leading to changes in the structure parameters. Zh. Tekh. Fiz. 69, 132–134 (June 1999)     

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.     

Isomorphous phase transition and orientational freezing in hexagonal mixed crystal C 70(1 - x) C60x. A pseudospin model

A four-state pseudospin model is constructed for the isomorphous phase transition hcp-2↦hcp-1 in pure C₇₀ and in C₇₀-rich mixed crystal C _{70(1 - x)} C_60x. With the specific anisotropic pseudospin interactions adapted to the C₇₀ crystal the model is equivalent to a two-state Ising model in a temperature-dependent field. Replica symmetric state of the model is shown to approach the critical point when the width of distribution of random fields and/or of random bonds increases. The temperature of the phase transition and the phase equilibrium temperature then are practically constant, whereas the experiment shows their strong decrease with x. The main effect of dilution resides in an x-dependence of the model parameters. Dilatometric data on the hexagonal C _{70(1 - x)} C_60x are used to fit these parameters. A metastable disordered phase subsisting below the phase transition is discovered in a range of the model parameters and is shown to be responsible for the macroscopic behaviour of the system. A good agreement with experimental data is obtained for the spontaneous strain and for the x-dependence of the hysteresis. Received 20 April 2001 and Received in final form 26 September 2001     

High-pressure crystal structure of thorium disulfide and diselenide and uranium disulfide

Abstract

The crystal structure of ThS₂, ThSe₂ and US₂ has been investigated for pressure up to 60GPa using x-ray powder diffraction. The bulk moduli are 175(10), 155(10) and 155(20) GPa, respectively. A pressure-induced phase transformation occurs at about 40 GPa for ThS₂, 30 GPa for ThSe₂ and 15 GPa for US₂. The results for ThSe₂ indicate that its high-pressure phase has a monoclinic structure. The same structure is compatible with the observed high-pressure spectra of ThS₂ and US₂. However, the crystal system assignment is less certain for these compounds.     

Structure of Quinoline-4-Carboxaldehyde-2,4-Dinitrophenyl-N Methylhydrazone

Abstract

The crystal structure of the title compound, C₁₇H₁₃N₅O₄, has been determined by single crystal x-ray diffraction at room temperature. The molecule is not planar, with dihedral angles of 7.2(1)° between the quinoline ring and N-methylhydrazone group, and 17.45(2)° between the N-methylhydrazone group and the phenyl ring. The crystal parameters of this compound are as follows: monoclinic P 21/n, a=9.525(2)Å, b = 15.192(2) Å, c = 11.302(2) A, β = 94.722(3)°, V = 1629.8(6) ų, Z = 4, Dx = 1.432 g/cm³, F(000) = 728, λ (MoKα) = 0.71070 Å, μ = 0.106 mm^?1, R_int = 0.017. The structure was solved by SHELXS-86 and refined by SHELXL-93. R = 0.07 for 2438 observed reflections with I > 2σ (I).     

Gd替代YBa2Cu3O7－δ超导体的相结构与局域电子结构的研究

利用正电子湮没实验，结合x射线衍射(XRD)结构分析，研究了具有混合稀土特征的(Y_{1 -x}Gd_x)Ba₂Cu₃O_7-δ系列样品. XRD 实验结果表明，半径较大的Gd离子Y位替代使得样品晶胞参数和晶胞体积增大，但所有样品 仍保持与YBa₂Cu₃O_7-δ(YBCO)样品相同的单相正交结 构. 正电子湮没实验表明，正电子各寿命参数表现出很强的Gd替代依赖关系. 从正电子实验 结果出发，计算了Cu-O链区局域电子密度n_e的变化. 结果表明，局域电子密度n _e随Gd含量x的增加而减小，而超导转变温度T_c随局域电子密度ne的减小而增加，这种局域电子密度n_e与超导电性的关联是与铜位替代 完全不同的，且可能是近年来人们关于混合稀土铜氧化物体系具有较高临界电流密度的原因 之一. 该研究结果为铜氧化物超导体的应用和机理研究提供了相应的正电子实验资料. 关键词： 超导电性 正电子湮没 相结构 局域电子密度     

Optical activity of incommensurate crystals with plane-wave modulation

A phenomenological approach is used to discuss the propagation of polarized light in the incommensurately modulated phases of crystals of the family A₂BX₄ with spatially averaged inversion symmetry. The Jones-matrix technique for an anisotropic inhomogeneous medium is used to calculate the optical parameters of a crystal with sinusoidally modulated dielectric parameters. It is demonstrated that these crystals can exhibit a weak optical activity, i.e., weaker than that of crystals without inversion symmetry. Also discussed are boundary conditions on the phase of the modulation wave at the surface of a crystal plate. Results are obtained suggesting that the optical properties of an incommensurately modulated crystal need not depend on the shape of the modulation wave. Fiz. Tverd. Tela (St. Petersburg) 39, 1360–1365 (August 1996)     

Low-temperature phase transition in Ba2TiGe2O8

Previous work on Ba₂TiGe₂O₈ crystals has shown an unusual low-temperature (～ 223 K on cooling, ～ 273 K on heating) phase transition. Precession x-ray photographs on Ba₂TiGe₂O₈ single crystals show an incommensurate modulation along b*, and, for the first time, also along a*. Single crystal intensity data confirm the average structure in space group Cmm2. There is positional disorder in the pyrogermanate groups, and this is the probable cause for the modulated structure. The low-temperature phase transition is proposed to be a lock-in transition, with the modulation along a* locking in at a value of 1/3. Several properties, as well as other unusual features of the low-temperature phase transition, are discussed in light of the proposed lock-in transition. Domain studies show that the ferroelastic domains are unstable in the low-temperature phase.     

A high-pressure study of Th3P4 and some U3X4 compounds

Abstract

The high-pressure crystal structures of Th₃P₄ and U₃X₄, where X = P, As and Sb, have been studied by means of synchrotron x-ray diffraction in the pressure range up to 50 GPa. The cubic phase of these compounds is retained in the whole pressure range. The bulk modulus B₀ and its pressure derivative B₀’ have been determined for each compound. A log-log plot of B₀ versus unit-cell volume gives a straight line for the uranium pnictides, with a slope about -5/3.     

Interaction of high-power x rays with sapphire crystals and quartz based materials

The interaction of high-power (10¹¹ W/cm²) soft (1–3 keV) x-rays with inorganic oxides (Al₂O₃ and SiO₂) is studied. It is found that when the wavelength of the x rays is comparable to the lattice constant of the crystal, besides generation of a high concentration of hot electrons and holes, there is broadening of the 2 p O²⁻-subband in the upper valence band owing to the local action of the strong x-ray field on regular oxygen sites. As a consequence, depending on the intensity of the x-ray pulses, a broadening of the fast (<1 ns) core-valence x-ray luminescence spectra is observed. Fiz. Tverd. Tela (St. Petersburg) 39, 286–289 (February 1997)     

Observation of standing x-rays in Bragg diffraction on high-T c superconducting crystals Nd1.85Ce0.15CuO4−δ

Curves of the secondary-fluorescence yield from the constituent elements of high-T _c superconducting crystals Nd_1.85Ce_0.15CuO_4−δ are measured in the dynamic x-ray diffraction regime. The form of the curves attests to the appearance of a standing x-ray wave in the crystal. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 9, 703–706 (10 May 1997)     

Single crystal diffraction with X-rays and neutrons: High quality at high pressure?

Abstract

Single crystal X-ray and neutron diffraction is essential for determining occupancies, positional as well as static and dynamic displacement parameters in crystalline matter by measuring Bragg, satellite or diffuse reflections. In our contribution a new low-temperature high pressure cell for neutron single crystal diffraction will be presented. It is designed to operate from a few K to ambient temperature in “orange” cryostats at pressures up to at least 3 GPa. We will present first neutron diffraction results obtained at E4/HMI Berlin and discuss the quality of the data and the significance of the results. A software package has been written (as a part of the PROMETHEUS system), which deals with the data reduction for both X-ray and neutron high pressure cell single crystal data. Likewise a data collection program has been developed for single crystal data collection on four-circle diffractometers using Merrill-Bassett cells. A series of single crystal experiments on H₂O and D₂O ice VI and KDP (KH₂PO₄) show that results of very high quality can be obtained routinely including even higher order terms in the atomic displacement parameters.     

Influence of Diffusion Parameters on the Spectral Characteristics of Raman Modes of Titanium-Diffused Lithium Niobate Planar Waveguides

ABSTRACT

A systematic investigation on the influence of the diffusion parameters (time and temperature) and initial titanium film thickness on the spectral characteristics of the LiNbO₃ Raman modes is reported. Raman spectra are measured in the range 50–1000 cm^?1 ～2 µm below the surface of the crystals. Broadening of the Raman lines and, therefore, crystal lattice disorder induced by the titanium ions are found to depend on the fabrication parameters. The disorder associated with the titanium ions near the surface of LiNbO₃ is encoded in the broadening of the A₁(TO₁) Raman line. A linear relation between the A₁(TO₁) mode broadening and the Ti concentration is presented. The diffusion theory is used to explain the experimental data. Raman spectroscopy combined with diffusion theory can be used to estimate the evolution of the titanium surface concentration.     

Effect of pressure on structure and fluorescence of SrFCL: Sm2+

Abstract

Lattice parameters of SrFCl have been measured by powder x-ray diffraction up to 38 GPa. Furthermore, energy levels and crystal field parameters of Sm²⁺ in SrFCl have been determined from fluorescence spectra at 100 K under pressures up to 10 GPa.