Analysis of X‐ray extinction in crystals with inhomogeniously distributed dislocations

The secondary extinction theory of Zachariasen for mosaic crystals as well as the formalism of Becker&Coppens have been used for substructure analysis (lattice disorientations, block size, density of excess dislocations, etc.) in crystals with inhomogeniously distributed dislocations in the Bragg case of diffraction geometry. In the case of large crystals D_S ≫ Λ_hkl the mean total density ρ_D of randomly distributed dislocations was also estimated taking into account additionally the primary X‐ ray extinction treatment. In this connection two cases are considered: I) pure secondary extinction related to arrangement of dislocation walls and II) mixed extinction in crystals with large subgrains related to randomly distributed dislocations and arrangement of dislocation walls. In order to check the considerations, the experimental and calculated data were compared for Be (model I) and Cu (model II) single crystals. The weakly distorted single crystals of Be and Cu were experimentally investigated in the Bragg case of diffraction geometry using Cu Kα₁ radiation by means of double crystal diffractometer. A new experimental procedure was proposed. Using alternative technique for substructure analysis (for instance EBSD) the reliability of analysis based on extinction phenomenon in weakly distorted single crystals has been checked. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

X-ray crystal orientation determination for semiconductor single crystals

An improved method to measu reorientation angles of flat single crystals was developed taking Buerger precession technique. The method is described and compared with Laue- and goniometer techniques. Advantages are low costs and short measuring time. An easy evaluation of the records allows to take this method as a routine angle determination for semiconductor manufacturing.     

Hydrothermal Synthesis of Zeolite Crystals Using Natural Scolecite

Natural scolecite is converted into analcime type zeolite by hydrothemal method at 180 °C at autogeneous pressure for 24 to 160 hours growth runs. Mordenite is detected in the final product. Cube and spheroid crystals of the products are illustrated and described. Crystals synthesized are characterized by X-ray diffraction, infrared spectroscopy EDAX and scanning electron microscopy. Particle size is estimated as 20–60 microns, induction period for nucleation as 12 hrs, and lattice parameter of cube crystals as 13.70 Å. Mechanism of growth, influence of anions and duration of growth etc. are discussed. It is established that the crystals grow in the system Na₂O Al₂O₃ SiO₂ H₂O.     

Influence of internal radiation on the heat transfer during growth of YAG single crystals by the Czochralski method

Heat and mass transfer taking place during growth of Y₃Al₅O₁₂ (YAG) crystals by the Czochralski method, including inner radiation, is analyzed numerically using a Finite Element Method. For inner radiative heat transfer through the crystal the band approximation model and real transmission characteristics, measured from obtained crystals, are used. The results reveal significant differences in temperature and melt flow for YAG crystals doped with different dopands influencing the optical properties of the crystals. When radiative heat transport through the crystal is taken into account the melt‐crystal interface shape is different from that when the radiative transport is not included. Its deflection remains constant over a wide range of crystal rotation rates until it finally rapidly changes in a narrow range of rotation rates. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electron microscopy of phase and structural transformations in soft magnetic nanocrystalline Fe-Zr-N films

The effect of deposition conditions (film thickness) on the structure of soft magnetic Fe_80–78Zr₁₀N_10–12 films formed by reactive magnetron deposition on a heat-resistant glass substrate has been investigated by analytical transmission electron microscopy, high-resolution electron microscopy, and diffraction analysis. The processes of evolution of the phase and structural state of films and the film-substrate interface upon annealing in the temperature range of 200–650°C have been analyzed taking into account the thermodynamic, kinetic, and structural factors and the specific features of the nanocrystalline state.     

PMA‐b‐PAA‐controlled synthesis of one‐dimensional CaCO3 superstructures

Crystallization of calcium carbonate (CaCO₃) crystals by a gas‐liquid diffusion method has been carried out in aqueous solution using a double‐hydrophilic block copolymer (DHBC) poly(maleic anhydride)‐b‐poly(acrylic acid) (PMA‐b‐PAA). The as‐prepared products were characterized with X‐ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM), selected area electron diffraction (SAED), high‐resolution transmission electron microscopy (HRTEM) and infrared spectroscopic analysis (FT‐IR). Uniform one‐dimensional calcite micro/nanostructures with different morphologies are fabricated through an assembled process. The influence of PMA‐b‐PAA copolymer concentration on the morphology of calcite nano/microwires is investigated, which plays an important role in the morphological control of building blocks composed of one‐dimensional calcite crystals. The possible formation mechanism of one‐dimensional CaCO₃ crystals was discussed. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Symmetry old and new: Some recent examples of the role of symmetry in X-ray analysis

Two recent examples in X-ray structure analysis at opposite ends of the molecular weight range, in which very careful consideration of space group and diffraction symmetry was necessary to resolve all the associated problems, are described. Case 1 involves the structures of (1) transdiBromo (1,4,8,11-tetraazacyclotetradecane) chromium (III) bromide and (2) the corresponding trans-bromo-chloro compound. Both crystallise in space groupP4₂/m with unit cell dimensions sufficiently close to suggest that the structures are isomorphous. It is shown however that the structures are not superimposable or even related by simple rotation or inversion, but that it is necessary to apply a hypothetical glide operation in order to bring the structures into coincidence. The organic moieties are thus structural enantiomorphs in spite of existing in a centrosymmetric space group. Case 2 involves studies on a co-crystallised derivative of the enzyme RNase with deoxycytidylyl-3–5-deoxyguanosine (dCpdG). Preliminary X-ray precession photographs, from low quality crystals, indicated an orthogonal C-centred unit cell, but were unable to define the true diffraction symmetry, which wasnot orthorhombic. The true symmetry was discovered only after solving the structure in a second (erroneous) cell and careful inspection of the lattice and diffraction symmetry from the measured intensity values.     

Precipitates of MnSi cubic phase in tetragonal Mn4Si7 crystal

Higher manganese silicides (HMSs) exhibit interesting thermoelectric and optoelectronic properties. Development of HMS-based thermoelements and microthermopiles of different designs may meet a number of problems, which can be solved only when the real structure of crystals and thin layers on which they are based is established. We have applied scanning and transmission electron microscopy and electron diffraction to investigate HMS crystals of two types: single crystals grown from melt by the Bridgman method and microislands formed by reactive diffusion during manganese vapor deposition on silicon substrates. The exact phase composition of these materials is established: matrix HMS crystal belonging to tetragonal system (Mn₄Si₇ composition) and precipitates of cubic manganese monosilicide MnSi. The shape and sizes of precipitates are determined, the crystallographic relationships between the tetragonal and cubic phases are found, and the interface is investigated.     

Exaggerated grain growth in bixbyite via fast diffusion along planar defects

Naturally occurring bixbyite single crystals from the Maynard claim at Thomas Mountain Range in Utah, USA, show pronounced grain growth, when compared to bixbyite crystals from other locations in close vicinity. These enlarged single crystals reveal characteristic linear surface features, which were initially interpreted as twin boundaries. HRTEM studies in conjunction with EDS analysis and electron diffraction, however, confirmed that these features originate from thin interlayers of braunite, Mn₇SiO₁₂, epitaxially grown within the host crystal. A model for the observed exaggerated grain growth is presented, which is based on the assumption that fast diffusion occurs along the braunite/bixbyite phase boundaries in three dimensions and, more importantly, promote nucleation at surface edges. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Enhancement of several Kikuchi lines in the vicinity of spot reflections

Kikuchi electron diffraction patterns of silicon have been obtained which demonstrate the simultaneous enhancement of several excess Kikuchi lines near different point reflections. This enhancement is explained within the elementary mechanism of the formation of Kikuchi patterns while taking into account the Kikuchi electron double diffraction.     

X-ray quantitative determination of the phases in a batch for the synthesis of poly- and monocrystal yttriumaluminium garnet

For the synthesis of ceramics and single crystals of yttriumaluminium garnet, a batch consisting of Y₂O₃ and Al₂O₃ is first treated thermally in order to obtain the garnet phase Y₃Al₅O₁₂. It is possible to make a choice of the optimum technological parameters of the batch if the amounts of all oxide phases, present during the synthesis, are well known. A combined X-ray quantitative method is suggested for this purpose. The initial oxides and a garnet phase are determined by the method with external standard. The rest two impurity oxide phases are determined as binary system. The specific diffraction characteristics of the present compounds were taken into account. The amounts of all phases in the batch, treated thermally from 1373 to 2075 K at every 100 K are shown.     

Synthesis of gallium nitride by ammonia injection into gallium melt

Gallium nitride (GaN) was synthesized by injecting ammonia gas into molten gallium at 900–980°C under atmospheric pressure. A large amount of GaN powder was reproducibly obtained using a simple apparatus. The synthesized powder was characterized by scanning electron microscopy, X-ray diffraction, photoluminescence and energy dispersive X-ray spectroscopy, and was found to consist of fine crystals of hexagonal GaN of good quality. The total of GaN obtained was far more than the amount calculated from expected saturation solubility in the Ga melt at that temperature. We speculate that the GaN crystals were largely formed by direct reaction between Ga and the gaseous N source at the surface of the NH₃ bubbles in the melt. GaN synthesized by this method may be useful as a starting material for bulk growth.     

Effect of the ultrasonic frequency on the multiple X-ray scattering from a LiNbO<Subscript>3</Subscript> crystal modulated by a surface acoustic wave

The effect of multiple scattering on the formation of the θ-2θ scan curves for a crystal modulated by a surface acoustic wave (SAW), depending on the ultrasonic frequency, has been investigated in the frame-work of the dynamical theory of X-ray diffraction. A model of a Rayleigh surface wave has been analyzed as applied to X-ray diffraction with allowance for the transverse and longitudinal elastic lattice strains. Using the example of the 127° Y′ cut of the LiNbO₃ crystal, it is established that the effects of multiple scattering can be neglected for ultrasonic frequencies above 650 MHz; this finding significantly simplifies the numerical calculations of X-ray diffraction from a crystal modulated by a short-wavelength SAW. A comparative quantitative analysis of the experimental data on synchrotron scattering from the 127° Y′ cut of a LiNbO₃ crystal modulated by a 952-MHz SAW have been performed, both taking into account and neglecting the effects of multiple scattering. It is shown that the computation time can be reduced by 2 to 3 orders of magnitude.     

Mechanism of formation of curved Kikuchi lines

The mechanism of formation of curved Kikuchi lines, observed at displacement of point reflections from their normal positions, is proposed. Curving of Kikuchi lines is explained taking into account the participation of diffracted electron beams in the formation of Kikuchi electron diffraction patterns.     

Discovery of difference in the diffuse scattering from nonstoichiometric Ca0.87La0.13F2.13 and Ca0.92Er0.08F2.08 crystals with different types of clusters of structural defects

A new approach to application of diffuse scattering (DS) for studying the defect structure of crystals on laboratory radiation sources is implemented. The basic principles of this approach are as follows: (i) choice of crystals with a high concentration of structural defects (highly nonstoichiometric Ca_{1 ? x} R _x F_{2 + x} phases), (ii) application of intermeasurement minimization method (experimental comparison) to select a weak desired DS signal from a superposition of signals of different nature, and (iii) choice of the basic model proceeding from the reliable information provided by accurate analysis of the contribution of structural defects to Bragg reflections. Significantly different DS diffraction patterns have been recorded for Ca_0.87La_0.13F_2.13 and Ca_0.92Er_0.08F_2.08 crystals, characterized by different types of structural-defect clusters, determined from the Bragg diffraction data. Experiments performed at 90–100 K proved that DS is caused by clusters with stable atomic configurations rather than cooperative thermal atomic vibrations. A set of methods is proposed which can efficiently be used in diagnostics of nanomaterials.     

The Twinned Crystal Structure of Zinc(II) Acetylacetonate Trimer

Abstract  The crystal structure of the title compound, C₃₀H₄₂O₁₂Zn₃, originally determined from untwinned crystals (Bennett et al. Acta Cryst B 24:904, 1968) has been redetermined from twinned crystals. The effect of the twinning is that additional reflections appear in the diffraction pattern leading to a unit cell with a too long c-axis in which the structure cannot be solved. Thus, for a successful structure solution the correct unit cell has to be found and for refinement the twinning has to be taken into account. The central Zn atom is located on a twofold rotation axis. It is hexacoordinated in a distorted octahedral mode, whereas the coordination geometry of the two terminal Zn atoms is distorted trigonal bipyramidal. Index Abstract  The crystal structure of the title compound, Zinc(II) acetylacetonate trimer, has been redetermined from twinned crystals. For a successful structure solution the correct unit cell had to be found and for refinement the twinning had to be taken into account. The central Zn atom is located on a twofold rotation axis. It is hexacoordinated in a distorted octahedral mode, whereas the coordination geometry of the two terminal Zn atoms is distorted trigonal bipyramidal. An erratum to this article can be found at     

Chemical Segregation in Vertical Bridgman Growth of GaInSb Alloys

A set of experiments on the solidification of Ga _1-x In _x Sb alloys with a large variation of the sample diameter (from 1mm to 10mm), of the growth rate (from 0.7 to 7 μm/s) and of the concentration (from x=0.01 to x= 0.1) is described. The associated radial and longitudinal segregation of the In have been analysed by SIMS or electron microprobe. Numerical simulation of the experiments, taking into account thermal, hydrodynamic and chemical behaviour has been carried out with the help of FIDAP. It is shown from these numerical results that a plateau of concentration can be reached even if a convective loop is present close to the interface, provided that the convection does not extend into the bulk of the liquid. This is in full agreement with the experimental results obtained. Supporting this analysis, in some experiments, a defect in term of verticality of the crucible led to complex 3-D convection involving the whole liquid, and in that case no plateau was obtained. For the radial segregation ΔC_R, three regimes of transport are found, characterised by the convective level: - A diffusive one, with a low, constant, ΔC_R related to the interface curvature. - A quasi-diffusive one (weak convection) in which ΔC_R increases with convection. - A convective one in which it decreases. Measured radial segregations are in good agreement with predictions from the numerical simulations.     

Crystal structure and phase transitions in the new crystals of [(CH3)2NH2]2CuCl4[(CH3)2NH2]Cl

Crystals grown from a solution of dimethylammonium and copper chlorides are studied using electron paramagnetic resonance (EPR) and X-ray diffraction. The dielectric properties of the crystals grown are measured. It is established that the crystals have the composition [(CH₃)₂NH₂]₂CuCl₄[(CH₃)₂NH₂]Cl and, in phase I at room temperature, are described by the orthorhombic space group Pna2₁ with the unit cell parameters a = 11.338 Å, b = 9.981 Å, and c = 15.675 Å. At temperatures of 279 K and 253 K, the crystals undergo jumpwise phase transitions into the incommensurate modulated ferroelectric phase II and commensurate modulated phase III, respectively. __________ Translated from Kristallografiya, Vol. 49, No. 1, 2004, pp. 92–100. Original Russian Text Copyright ? 2004 by Kirpichnikova, Pietraszko, Bednarski, Waplak, Sheleg. Dedicated to the 80th Birthday of L.A. Shuvalov     

Devitrification of Ni-based glassy alloys containing noble metals in relation with the supercooled liquid region

The formation of the supercooled liquid region and devitrification behavior of Ni-based glassy alloys were studied by using X-ray diffraction, transmission electron microscopy, differential scanning calorimetry and isothermal calorimetry. oC68 Ni₁₀Zr₇-type phase is primarily formed in the studied alloys in the initial stage of the devitrification process by nucleation and three-dimensional diffusion controlled growth. The replacement of Cu by Ni in Cu₅₅Zr₃₀Ti₁₀Pd₅ glassy alloy induces precipitation of oC68 Ni₁₀Zr₇ phase directly from the glassy phase. The reasons for such a behavior are discussed taking into account mixing enthalpy in a liquid state and the interval of the supercooled liquid region.     

Nonidentified Kikuchi lines with reverse contrast

Electron diffraction patterns of silicon in transmission with contrast reversal from bright to dark for an unidentified Kikuchi line along its length have been obtained. The contrast reversal of an unidentified line is explained within the elementary mechanism of Kikuchi pattern formation taking into account the Kikuchi electron double diffraction.