X-ray diffraction analysis and occurrence of multiple phases in Bi-Sr-Ca-Cu-O superconductors

Starting composition 1112 for Bi-Sr-Ca-Cu-oxide yields multiphase super-conductors with the proportion of constituent phases depending sensitively on the annealing temperature. The R-T curves show zero resistivity and the transition corresponding toT _c = 80 K phase prominently. However, indexing of X-ray diffraction peaks reveals presence of 80 K (lowT _c) as well as 108 K (highT _c) phase. The lowT _c phase thus corresponds to the orthorhombic structure with a unit cell ofa = 5.4Å,b = 27 Å andc = 30.56 Å. This is further understood to be composed of a pseudotetragonal cell ofa =b = 5.41 Å. The highT _c phase similarly pertains to the orthorhombic structure withc = 36 Å.     

Structural transformations of K<Subscript>3</Subscript>H(SO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystals with variations in temperature

Single crystals of the K₃H(SO₄)₂ compound are investigated using X-ray diffraction on Xcalibur S and Bruker diffractometers. The structure of the low-temperature monoclinic phase is refined (space group C2/c, z = 4, a = 14.698(1) Å, b = 5.683(1) Å, c = 9.783(1) Å, β = 103.01(1)°, T = 293 K, Bruker diffractometer), the structural phase transition is revealed, and the structure of the high-temperature trigonal phase is determined (space group R \(\bar 3\) m, z = 3, a = 5.73(1) Å,c = 21.51(1) Å,T = 458 K, Xcalibur diffractometer).     

Crystallographic and vibrational study of hexanitroethane

The crystal structure of hexanitroethane has been determined in the low-temperature ordered phase at 145 K (P2₁/c: a = 10.152(2) Å, b = 9.311(2) Å, c = 10.251(2) Å, β = 97.54(1)°, V = 960.6(3) ų, Z = 4 with 2 non-equivalent molecules in the unit cell).The far-infrared, infrared and Raman spectra of hexanitroethane in both low-temperature ordered and high-temperature disordered crystal phases were recorded. The internal modes are interpreted with the help of normal coordinate analysis and a conformational analysis with the MNDO method. The transition mechanism is discussed.     

Thin ferroelectric Nd-doped BiFeO<Subscript>3</Subscript> films with orthorhombic structure

X-ray diffraction and Raman spectroscopy of epitaxial Nd-doped bismuth ferrite films on MgO substrates reveal their orthorhombic symmetry Fmm2 (a = 7.914 Å, b = 7.913 Å, and c = 7.937 Å).     

Structure parameters and polarity of the wurtzite type compounds Sic—2H and ZnO

The structure parameters of the wurtzite type compounds Sic—2H and ZnO were determined using X-ray intensities from single crystals collected with an automatic single crystal diffractometer. The lattice constants and the u-values are $\text{a}\text{= 3.079}\text{A}\text{?}\text{,}\text{c}\text{= 5.053}\text{A}\text{?}\text{,}\text{u}\text{= 0.3760}$ for Sic—2H and $\text{a}\text{= 3.253}\text{A}\text{?}\text{,}\text{c}\text{= 5.213}\text{A}\text{?}\text{,}\text{u}\text{= 0.3820}$ for ZnO. The final reliability values for refinements with anisotropic temperature factors are equal to R = 0.032 for Sic—2H and R = 0.024 for ZnO.     

Elastic constants and Debye temperature of wz-AlN and wz-GaN semiconductors under high pressure from first-principles

First-principles calculations were performed to study the elastic stiffness constants (C _ij ) and Debye temperature (?? _D) of wurzite (wz) AlN and GaN binary semiconductors at high pressure. The lattice constants were calculated from the optimized structure of these materials. The band gaps were calculated at Γ point using local density approximation (LDA) approach. The unit cell volume, lattice parameters, c/a, internal parameter (u), elastic constant (C _ij ), Debye temperature (?? _D), Hubbard parameter (U) and band gap (E _g) were studied under different pressures. The bulk modulus (B ₀), reduced bulk modulus ( \(B_{0}^{\prime })\) and Poisson ratio (υ) were also calculated. The calculated values of these parameters are in fair agreement with the available experimental and reported values.     

A new crystalline HMX polymorph: ɛ-HMX

A new crystalline HMX polymorph, ?-HMX, was obtained. ?-HMX crystals were studied by X-ray structure analysis, optical microscopy, and differential scanning calorimetry. Their space group is P2₁/c. The unit cell parameters are a = 21.799(3) Å, b = 10.913(2) Å, c = 10.819(2) Å, and β = 97.43(2)°, V = 2552.15 ų, Z = 4. ?-HMX molecules are not equivalent in crystals and have chair conformations. The heat of the polymorphic transition of ?-HMX into the δ-polymorph was measured. The transition occurred with the intermediate formation of β-HMX. The dependence between the heats of polymorphic transitions and the densities of crystals of various HMX polymorphs was demonstrated. The character of this dependence was to a substantial extent determined by the type of HMX molecule conformation.     

AgTh2(PO4)3 — A new case of the non-hydrogen bonded phosphate ferroelectric

The basic crystallographic data of AgTh₂(PO₄)₃ single crystals have been determined for the first time by means of X-ray diffraction methods. The crystals are monoclinic, space group isCc with four formula units per unit cell. The dimensions of the unit cell are:a=17·385 Å,b=6·815 Å,c=8·148 Å,β=101·10°. Using the Sawyer and Tower method it has been proved that the crystals possess ferroelectric properties. Performing the measurements at room temperature the values of spontaneous polarization and coercive field in the direction normal to (100) face have been determined.     

Syntheses and structure characterization of inorganic/organic coordination polymers: Ag(dpa), Co(O3PH)(4,4′-bpy)(H2O), Zn(O3PH)(4,4′-bpy)0.5 and Mn[O2PH(C6H5)]2(4,4′-bpy) (dpa=2,2′-dipyridylamine; 4,4′-bpy=4,4′-bipyridine)

Four novel coordination polymers: Ag(dpa) I, Co(O₃PH)(4,4′-bpy)(H₂O) II, Zn(O₃PH)(4,4′-bpy)_0.5 III and Mn[O₂PH(C₆H₅)]₂(4,4′-bpy) IV (dpa=2,2′-dipyridylamine; 4,4′-bpy=4,4′-bipyridine), were synthesized by microwave heating and characterized by X-ray crystallography. I crystallizes in monoclinic space group P21/n with a=11.576(2) Å, b=5.585(2) Å, c=15.243(4) Å, β=109.00(2)°, V=931.8(3) ų. II crystallizes in monoclinic Cc space group with a=22.477(7) Å, b=5.280(1) Å, c=10.404(4) Å, β=96.08(3)°, V=1227.8(7) ų. III crystallizes in monoclinic P21/c space group with a=9.758(2) Å, b=7.449(3) Å, c=10.277(2) Å, β=100.02(2)°, V=735.6(4) ų. IV crystallizes in monoclinic space group P2/c with a=10.174(1) Å, b=11.817(3) Å, c=18.784(4) Å, β=102.14(1)°, V=2207.8(8) ų. I consists of linear metal–metal chains wrapped by dpa ligands. II and III consist of two-dimensional M^II(O₃PH) inorganic sheets cross-linked by 4,4′-bpy ligands, while IV is formed by Mn[O₂PH(C₆H₅)]₂ sheets cross-linked by 4,4′-bpy ligands. I exhibits two-step thermal decomposition at ~200 and ~250°C, resulting in the reduction of Ag⁺ to Ag metal. II loses its coordination water at ~100°C, leaving vacant coordination sites at Co²⁺ ions, while the original framework remains intact. The removal of 4,4′-bpy in II–IV occurs at elevated temperatures above 250, 200 and 400°C respectively.     

Structure,microstructure, and thermopower of highest manganese silicide crystals grown from zinc solution-melt

MnSi_1.71–1.75 single crystals are grown using the combination of solidification from solution-melt and the Bridgman crystal growth technique. The single crystals represent tetrahedral bipyramids and correspond to the highest manganese silicide (HMS) modification with lattice parameters a = b = 5.52(1) Å and c = 65.7(8) Å. The thermopower measured between the parallel faces of these single crystals is anomalously high for plain HMS (180–190 μV/K).     

Electron paramagnetic resonance in deuterated nickel fluosilicate

Electron paramagnetic resonance measurements in single crystals of NiSiF₆. 6D₂O were made at K, Ku and Ka bands at 4.2 K and between 77 K and 300 K. The measured g values were in the range 2.23–2.26, while the zero-field splitting parameter D varied from ?(0.185 ± 0.005) cm^?1 at 4.2 K to ?(0.53 ± 0.01) cm^?1 at 298 K. The parameters of the trimolecular hexagonal unit cell were determined to be approximately a = 9.28 Å, c = 9.58 Å from powder X-ray diffraction measurements at room temperature.     

X-ray topography, photopyroelectric and two-photon absorption studies on solution grown benzimidazole single crystal

Benzimidazole (BMZ) is an organic nonlinear optical material, and it is possible to grow it in the laboratory by a slow evaporation solution growth technique using methanol as the solvent. Its nonlinear optical harmonic generation efficiency is 4.5 times higher than that of standard potassium dihydrogen phosphate (KDP) single crystal. It crystallizes with orthorhombic structure, which is noncentrosymmetric in nature, with lattice parameters a=13.58 Å, b=6.85 Å, c=6.97 Å. The functional groups of the grown specimen are confirmed by heteronuclear chemical shift correlation (HETCOR) analysis and it is found that other elements are not incorporated in the compound. The crystalline perfection is examined by high resolution X-ray diffraction (HRXRD), and its topographic image was recorded for the as-grown single crystal indicates the presence of some grain boundaries in the grown specimen. Thermal parameters such as thermal diffusivity (α), thermal conductivity (k), thermal effusivity (e), and heat capacity (c _p ) have been determined by a photopyroelectric thermal wave method. In addition, piezoelectricity, birefringence, and third-order nonlinear optical properties have been examined. The two-photon absorption coefficient of the title compound is determined by the open-aperture Z-scan technique.     

Thiourea-doped ammonium dihydrogen phosphate: A single crystal neutron diffraction investigation

Thiourea-doped ammonium dihydrogen phosphate (TADP) exhibits nonlinear optical property and the second harmonic generation efficiency of these crystals is three times that of pure ammonium dihydrogen phosphate (ADP) crystal. In this context, the study of structural distortion in the thiourea-doped ADP crystal is significant, hence single neutron diffraction investigations were undertaken. The final R-factors are: R[F ² > 2σ(F ²)] = 0.11, Goodness of fit(S) = 1.15. Though the dopant could not be located from the difference Fourier map, the cell parameters (a = b = 7.531(3) A, c = 7.544(5) Å) were found to be significantly greater than that of pure ADP at RT (a = b = 7.502(1) Å, c = 7.546(1) Å). This indicates that the dopant concentration in the crystals is small but enough to bring changes in the overall average structure.     

Structural transformation in Mg2NiH4

X-Ray diffraction measurements show that on heating Mg₂NiH₄ in a 1 atm pressure H₂ atmosphere, above ～250°C it transforms into a cubic structure, metal atoms in CaF₂ arrangement, a = 6.525 Å. It is concluded that the H atoms are in tetrahedral clusters, and that the structure is only weakly ionic. This conclusion is also supported by NMR measurements. The 20°C structure of Mg₂NiH₄ is shown to be describable primarily as a slight monoclinic distortion of the cubic unit cell; a = 6.594 Å, b = 6.412 Å, c = 6.490 Å and β = 93.1°. However, weak small angle lines show that a longer range order exists and that the true unit cell, which we have not determined, must be very large. To what extent the cubic phase should be considered a high temperature and/or low concentration (Mg₂NiH_4??) phase is not resolved.     

Synthesis,growth and characterization of L-proline dimercuricchloride single crystals for frequency conversion applications

A semi-organic nonlinear optical L-proline dimercuricchloride (LPDMC) material has been synthesized. LPDMC single crystals were grown from aqueous solution by a slow cooling method. Good quality single crystals of size 19×6×3 mm³ have been grown over a period of 3 weeks. The grown crystals have been subjected to single crystal X-ray diffraction analysis to determine the cell parameters. The title compound crystallizes in the triclinic system with a noncentrosymmetric space group P1 and with unit-cell parameters a=7.2742(4) Å, b=9.4472(5) Å, c=10.4767(6) Å, α=108.621(3)^°, β=107.260(2)^°, γ=97.353(2)^° and volume=631.51(6) Å³. Optical and dielectric properties of the crystals have been studied. The thermal stability of the crystals was determined by thermogravimetric analysis/differential thermal analysis. The second harmonic generation efficiency of the crystals was obtained by the classical powder technique using a Nd:YAG laser and it is found to be 2.5 times that of potassium dihydrogen phosphate.     

Ferromagnetic ordering in the MMnF6 series (M = Ni,Cu, Zn,Pd)

Neutron diffraction experiments performed on MMnF₆ compounds (M = Ni, Cu, Zn, Pd) have shown that both NiMnF₆ and ZnMnF₆ crystallize in the rhombohedral R$\text{3}$ space group with a cationic ordering, whereas PdMnF₆ shows cationic disordering (R$\text{3}$c, a = 5.46 Å, α = 54.45°). CuMnF₆ has a monoclinic distortion derived from the MnF₃ structural type (a = 8.57 Å, b = 4.85 Å, c = 13.46 Å, β = 92.12°). The magnetic properties can be correlated with the structure: the R$\text{3}$ ordered compounds show a ferromagnetic behavior (T_C(NiMnF₆) = 39 K; T_C(ZnMnF₆) = 9.5 K; T_C(CdMnF₆) = 8 K), while the other two have an antiferromagnetic ordering.     

Neutron diffraction determination of the magnetic structures of NpCo2Si2 and NpCu2Si2

A small polycrystalline ingot sample of NpCo₂Si₂ (weight ≈ 1.5 g) has been studied by neutron diffration between 2 and 160 K on the multi-detector D1B of ILL, Grenoble. At 100 K, the crystal structure is body-centered tetragonal (space group 14/mmm) with a = 3.886 Å and c =9.649 Å. Below T_N = (44 ± 2) K, seven superlattice lines are observed which correspond to a simple tetragonal lattice with lattice constants as above. They are consistent with a type I antiferromagnetic structure of the Np (2a) sublattice, with (001) ferromagnetic sheets coupled antiferromagnetically according to the sequence +-+-. At 6 K, the neptunium moment obtained from the diffracted intensities is: (1.48 ± 0.20)_μuB, and makes an angle 52° ± 15° with the c axis. The cobalt moment is certainly smallet than 0.3_μuB. The Np moment correlates well with the ²³⁷Np hyperfine field deduced from Mos?sbauer spectroscopy; the sublattice magnetization-temoperature curve follows very well the $\text{J=}\text{1}\text{2}$ brillouin curve. The magnetism is therefore probably of lovalized character in this compound. An isomorphous sample of NpCu₂Si₂ (a = 3.990 Å c = 9.920 Å) was shown to be ferromagnetic below (41 ± 2) K, with the Np moment [1.5 ± 0.2)_μuB] aligned along the c axis.     

Structure and electrophysical properties of ferrocobaltites <Emphasis Type="Italic">Ln</Emphasis>BaFeCoO<Subscript>5 + δ</Subscript> (<Emphasis Type="Italic">Ln</Emphasis> = Tb,Dy, Ho,Y)

The ferrocobaltites LnBaFeCoO^{5 + δ} (Ln = Tb, Dy, Ho, Y) have been synthesized, and the parameters of their crystal structure have been determined. The thermal expansion, electrical resistivity ρ, and thermopower S of the synthesized compounds have been investigated in air at temperatures in the range from 300 to 1100 K. The compounds have a tetragonal structure (symmetry space group P4/mmm) with the unit cell parameters a = 3.9000 Å and c = 7.5922 Å (Ln = Tb, δ = 0.31), a = 3.8973 Å and c = 7.5679 Å (Ln = Dy, δ = 0.34), a = 3.8970 Å and c = 7.5507 Å (Ln = Ho, δ = 0.28), and a = 3.9029 Å and c = 7.5538 Å (Ln = Y, δ = 0.25). The ferrocobaltites under investigation are p-type semiconductors, and their electrical resistivity ρ and thermopower S decrease in the sequence Tb → Ho → Y → Dy (at room temperature). The linear thermal expansion coefficient of the LnBaFeCoO^{5 + δ} phases in the vicinity of the temperatures ranging from 465 to 535 K increases from (1.15?1.23) × 10^?5 to (1.73?1.93) × 10^?5 K^?1. The parameters of charge transfer in these ferrocobaltites have been determined. It has been found that an increase in the temperature leads to an increase in the excitation energy of charge carriers and a decrease in the activation energy of charge carrier transfer.     

New crystalline phases of an equiatomic K-Cs alloy at low temperature

Solid equiatomic K-Cs alloys have been investigated by X-ray diffraction throughout the temperature range 300-100K. The results indicate that a phase separation occurs below 185K accompanied by the appearance of an ordered phase in this range. This phase has a hexagonal lattice with parameters: a = 9.32(1) Å and c = 11.80(2) Å (at 170K). Evidence from our other studies [7] indicates that its composition is K₂Cs. Another phase transformation in this ordered crystal is observed below 120K. There is no change of lattice symmetry but the unit cell constants shrink to the values: a = 9.11(1) Å and c = 10.86(2) Å (at 100K). The transformation can be ascribed to a rearrangement of the electronic structure of Cs.     

Si(111)衬底上生长有多缓冲层的六方GaN晶格常数计算和应变分析

利用卢瑟福背散射/沟道技术和高分辨率X射线衍射技术对在Si(111)衬底上利用金属有机化合物气相外延技术(MOVPE)生长有多缓冲层的六方GaN外延膜进行结晶品质计算、晶格常数计算和应变分析. 实验结果表明：GaN外延膜的结晶品质为χ_min=1.54%，已达到完美晶体的结晶品质(χ_min=1%—2%)；GaN外延膜的水平方向和垂直方向晶格常数分别为：a_epi＝0.31903nm，c_epi=0.51837nm，基本达到G 关键词： GaN 高分辨X射线衍射 卢瑟福背散射/沟道 弹性应变