High-resolution Diffractometer Research Articles

High-efficiency transmission neutron polarizer for high-resolution double crystal diffractometer

An efficient transmission geometry neutron polarizer for the high-resolution double crystal diffractometer at HMI ( λ = 4.8 A ̊ ) is described. A polarization of about 94% was achieved and the polarized neutron beam intensity amounts to 40% of the nonpolarized beam intensity. This opens up wide possibilities for the study of magnetic small-angle scattering for extremely small momentum transfer ( Q ∼ 10 −5 A ̊ −1 ).

Reciprocal space mapping

This review covers the recent advances in reciprocal space mapping. The experimental techniques as well as the theoretical and conceptual developments are discussed. The advantages of reciprocal space mapping over the conventional single scan X-ray scattering methods become clear from the examples presented. Extracting the additional information from mapping in reciprocal space maps has led to a deeper understanding of materials. Imperfect materials benefit enormously from these methods. Near perfect materials also indicate weak diffuse scattering that can now be interpreted in terms of defects, etc., whereas with single scans the influence is difficult to observe and separate from other features. Reciprocal space maps can be collected with both high and low angular resolution diffractometers, depending on the application, although a combination of resolutions may be necessary. It is also growing in importance in the analysis of materials using specular reflectometry. High-resolution reciprocal space mapping is not restricted to good crystalline quality. Examples of reciprocal space mapping are given for semiconductors, metals, ceramics and biological samples. For semiconductor materials, reciprocal space mapping has now become almost routine in the study of lattice relaxation in thin layers and in the assessment of the “quality” of materials. Combinations of mapping with topography and precision lattice parameter determination are also discussed. The latter part of this review discusses the advantages of three-dimensional reciprocal space mapping, which takes the analysis further. With this method the full three-dimensional shapes in reciprocal space can be studied.

Single-crystal diffraction study of phase separation in La 2CuO 4+δ

Single crystals of La 2CuO 4+δ with δ = 0.04 and δ = 0.04 have been studied in the temperature range 10 K ⩽ T ⩽ 293 K using high-resolution neutron diffractometer. In both crystals, the concentration of excess oxygen is inside the miscibility gap. However, the La 2CuO 4.03 crystal does not undergo a phase separation - homogeneous oxygen distribution was preserved over the entire temperature region studied - while in the La 2CuO 4.04 crystal, the phase separation phenomenon, connected with excess oxygen diffusion was observed. Our data provide direct evidence that both the oxygen-rich and oxygen-poor regions have the same Cmca space symmetry. The dimensions of the coherent regions of coexisting phases are (975 ± 22) Å in the (0 1 0) direction, and (1460 ± 160) Å in the perpendicular direction.

Partial improvement of crystal quality for microgravity-grown apocrustacyanin C1.

The protein apocrustacyanin C(1) has been crystallized by vapour diffusion in both microgravity (the NASA space shuttle USML-2 mission) and on the ground. Rocking width measurements were made on the crystals at the ESRF Swiss-Norwegian beamline using a high-resolution psi-circle diffractometer from the University of Karlsruhe. Crystal perfection was then evaluated, from comparison of the reflection rocking curves from a total of five crystals (three grown in microgravity and two earth controls), and by plotting mosaicity versus reflection signal/noise. Comparison was then made with previous measurements of almost 'perfect' lysozyme crystals grown aboard IML-2 and Spacehab-I and reported by Snell et al. [Snell, Weisgerber, Helliwell, Weckert, Hölzer & Schroer (1995). Acta Cryst. D51, 1099-1102]. Overall, the best diffraction-quality apocrustacyanin C(1) crystal was microgravity grown, but one earth-grown crystal was as good as one of the other microgravity-grown crystals. The remaining two crystals (one from microgravity and one from earth) were poorer than the other three and of fairly equal quality. Crystal movement during growth in microgravity, resulting from the use of vapour-diffusion geometry, may be the cause of not realising the 'theoretical' limit of perfect protein crystal quality.

High-resolution X-ray diffraction from microstructures

Novel high-resolution X-ray diffractometers have been used to investigate a variety of microstructures such as twins, tweed, and domain boundaries in ferroelastic materials. It is shown that copious information can be extracted using our experimental methods. The instruments are equipped with 1D and 2D detectors, where the distance to the sample can be varied to enhance the intrinsic resolution. Both the specimen and the detectors possess several degrees of freedom which include sample tilts and rotations as well as translations. In our contribution we introduce briefly the experimental and theoretical methods connected with this special equipment and describe its application to the characterization of a wide variety of specimens such as single crystals, ceramic materials and thin films.

Anomalous X-ray scattering from bulk microdefects

X-ray anomalous scattering measured with a high-resolution triple-crystal diffractometer was analysed in terms of different models of microdefects present in the bulk of InP crystal. The observed broadening of the Laue reflections is consistent with the formation of large aggregations of dislocation loops belonging to {111}〈110〉 system. The complete simulation of the diffuse intensity including both Huang and Stokes-Wilson regions in comparison with the experimental data revealed that these aggregations consist of vacancy loops occupying predominantly the (111) plane and that the rest of the {111} planes are occupied by the interstitial loops.

Influence of Sample‐Movement on Particle‐Randomization of Powder Samples in the Parallel‐Beam Geometry

Today, from powder X‐ray diffraction the scientists want to obtain high resolution diffraction patterns with reliable Bragg‐reflection intensities. Two well‐known and closely connected obstacles on the way are texture and particle randomization of the sample, which strongly influence the measured intensities. In the work presented here, we examine the second problem.Especially with high resolution diffractometers, for well crystallized or highly absorbing samples the number of contributing crystals in the powder is too small, thus introducing significant errors in the profiles and measured intensities of Bragg‐reflections. This may be the major source of inaccuracy in data used for structure determination. Calculations of the error of the integrated intensities are presented, for the high resolution, parallel beam geometry at a synchrotron X‐ray source. Results exhibit errors of 40 % in the range of highest resolution for a sample of 3 μm crystals of corundum, with the sample at rest. To enhance randomization, several methods of sample‐movement are considered. A new effective method is proposed, where the rotation axis of the flat sample‐disk is slightly inclined out of the diffraction plane.

The effect of dislocation contrast on x-ray line broadening: A new approach to line profile analysis

The x-ray line profiles of an ultrafine grained copper crystal, produced by equal-channel angular pressing, were measured by a special high resolution diffractometer with negligible instrumental line broadening. The analysis of the line breadths and the Fourier coefficients have shown that taking into account the contrast caused by dislocations on line profiles gives new scaling factors in the Williamson–Hall plot and in the Warren–Averbach analysis, respectively. When strain is caused by dislocations the new procedure proposed here enables a straightforward determination of particle size and strain, the latter in terms of the dislocation density.

Neutron diffraction investigation of phase separation in La2CuO4+y single crystals

La2CuO4+y single crystals with y=0.03 and 0.04 are investigated with a high-resolution neutron diffractometer in the temperature range 10<T<293 K. Althouogh the excess oxygen concentration y falls in the range of the dissolution gap, the crystal with y=0.03 does not undergo phase separation (PS)—the oxygen distribution in this crystal remains uniform over the entire experimental temperature interval. Macroscopic PS is observed in the crystal with y=0.04. An analysis of the dependence of the widths of the reflection orders on the interplanar separation gives the sizes of the coherent regions of the two coexisting phases: (970±30) A in the direction of the tetragonal axis c and (1460 645) A in the basal plane.

High-resolution X-ray diffraction analysis of magnetically aligned high- Tc superconducting ceramics

A large range of magnetically treated superconducting ceramics has been investigated using new high-resolution X-ray diffractometers in combination with different integration techniques to obtain quantitative and qualitative results on the degree of alignment. The diffractometers combine and enhance the flexibility of a 4-circle diffractometer with the possibility to use it as a bond camera and as grazing incidence surface diffractometer. The X-ray beam is strictly monochromatic (CuKα 1) and focused with a long focal length on the sample. The sample goniometer can be rotated and shifted around three perpendicular axes and two translation directions, respectively. The instruments are equipped with 1D and 2D detectors and their distance to the sample can be varied between 10 and 30 cm. In our contribution we describe the experimental methods connected with this special equipment, their application to the analysis of the degree of alignment in high T c superconducting ceramics and compare the results to those obtained by standard diffraction methods.

Residual strain in GaN epilayers grown on sapphire and (6H)SiC substrates

A high-resolution multiple-crystal reflection diffractometer was used to determine the residual strain in GaN epilayers grown on different substrates. Our results show that the GaN epilayers with thickness of about 1 μm are not fully relaxed. The strain situation in a GaN epilayer grown on SiC substrate is totally different from that in GaN layer on sapphire, which are the main reason for the scatter in the recent data on the optical properties of group III nitrides.

Local Lattice Parameter Changes in Monoerystalline Turbine Blades Subjected to Service-like Conditions

Two helicopter turbine blades of the monocrystalline nickel-base superalloy CMSX-6 which had been used in test engines under service-like conditions were investigated. While the microstructure was investigated by conventional methods of metallography and scanning electron microscopy (SEM), a special high-resolution X-ray double-crystal diffractometer was used in the present work in order to determine the local lattice parameter distributions. In some parts of the turbine blades directional coarsening of the originally cuboidal precipitates was found, similar to that known from the investigation of creep-deformed laboratory samples. The measurement of profiles of X-ray Bragg peaks at different points within the turbine blades yielded local changes of the lattice parameter distributions, induced by the exposure to operation conditions. These lattice parameter changes are connected with internal stresses. The estimated effects are compared with results obtained on samples creep-deformed under defined conditions. The results are important for the understanding of the stress distributions within the turbine blades during service.

Austenite content and dislocation density in electron-beam welds of a stainless maraging steel

The volume fraction of austenite and dislocation density in electron beam weld joints of a martensitic-austenitic age-hardenable CrNiMoTiAl stainless steel were investigated using neutron diffraction. Experiments were performed on a high-resolution diffractometer which resolved microstrains of the order of 10 −4. The effect of post-weld heat treatment on austenite content and dislocation density was investigated. As in other maraging steels, the maximum volume fraction of austenite was obtained after quenching and intercritical annealing. Dislocation densities computed on the base of mean-square microstrain values were about 10 11 cm −2. These densities were systematically higher in martensite than in austenite, with very small differences between the weld metal and base metal.

High-resolution neutron diffractometer for internal stress measurements

This contribution presents a recently developed project to build a new dedicated diffractometer for residual-stress determination at Laboratoire Léon-Brillouin (CEA-CNRS). Details of the project, in particular the use of a positionsensitive multidetector, are described. The expected performance of the new instrument, which is presently under construction, is discussed with reference to materials studies.

A comparison of techniques for nondestructive composition measurements in CdZnTe substrates

We report an overview and a comparison of nondestructive optical techniques for determining alloy composition x in Cd1-xZnxTe substrates for HgCdTe epitaxy. The methods for single-point measurements include a new x-ray diffraction technique for precision lattice parameter measurements using a standard highresolution diffractometer, room-temperature photoreflectance, and low-temperature photoluminescence. We compare measurements on the same set of samples by all three techniques. Comparisons of precision and accuracy, with a discussion of the strengths and weaknesses of different techniques, are presented. In addition, a new photoluminescence excitation technique for full-wafer imaging of composition variations is described.

Applications of multiple-crystal diffractometry

The elucidation of the structure of semiconductor multilayers can be adequately determined by X-ray methods but the interpretation is not always straightforward. In this paper we introduce the idea of full three-dimensional diffraction-space mapping to obtain information on the three-dimensional structure of imperfect materials. We also stress the importance of this method for the interpretation of the data from high-resolution X-ray diffractometry. The presence of defects and diffraction effects can create significant changes to the diffraction pattern that require a more complete analysis than that obtained from simple profiles. These subtle influences can in general only be understood by diffraction-space mapping. Interpretation of diffraction-space maps from the high-resolution multiple-crystal multiple-reflection diffractometer permits the use of three extra very powerful tools. The first is multiple-crystal topography so that the diffraction-space intensity features can be related to lateral contrast on the photographic emulsion, the second is the accurate determination of lattice parameters and the third is the simulation of the diffraction shapes using dynamical theory.

High-resolution X-ray characterization of periodically domain-inverted nonlinear optical crystals

A high-resolution triple-axis diffractometer has been used for the structural characterization of periodically domain-inverted nonlinear optical crystals of KTiOPO4 and LiNbO3. Striations have been revealed in high-strain-sensitivity multiple-crystal topographs of the domain-inverted regions of both these samples and these are dominated by orientation contrast. The combination of high-resolution reciprocal-space mapping and topography has shown that the extended diffraction streak in the q(210) direction for the domain-inverted LiNbO3 originates from the 'minutely misoriented structure' which is related to the original configuration of dislocations. The reason for the generation of the structural imperfections via the domain-inversion processing is interpreted in terms of the converse piezoelectric effect.

Structure of a metastable phase of piracetam from X-ray powder diffraction using the atom–atom potential method

The crystal structure of the room-temperature metastable phase of 2-oxo-1-pyrrolidineacetamide (piracetam), C 6 H 10 N 2 O 2 , has been solved using the atom-atom potential method and refined by Rietveld refinement from powder diffraction data collected with a curved position-sensitive detector (INEL CPS 120) using Debye-Scherrer diffraction geometry with monochromatic X-rays. In the first stage, the indexing of the powder pattern was performed by the successive dichotomy method from data collected with a high-resolution diffractometer using the Bragg-Brentano geometry. The lattice parameters are a = 6.747 (2), b = 13.418 (3), c = 8.090 (2) A, β = 99.01 (3)°, and the space group is P2 1 /n. The structure model was obtained from minimization of the crystal-lattice potential energy calculated with semi-empirical atom-atom potentials using the PMC [Packing of Molecules in Crystals; Dzyabchenko, Belsky & Zorkii (1979). Kristallografiya, 24, 221-226] program. The Rietveld refinement converged to the final crystal-structure model indicator R B = 0.04 and profile factors Rp = 0.03 and R wp = 0.04. The molecular packing is characterized by intermolecular hydrogen bonds (N-H...O). The structure consists of two types of perpendicular infinite chains of piracetam molecules, which build parallel layers containing rings. No cyclic dimers were found. For a description of the hydrogen-bond patterns, the method based on graph theory was employed

High energy scattering beamlines at European Synchrotron Radiation Facility

The scientific background and scope of research at the High Energy Scattering Beamlines of the European Synchrotron Radiation Facility are reviewed briefly. The beamline has two different insertion devices (IDs), a permanent magnet multipole wiggler and a superconducting wavelength shifter. The IDs can be used alternatingly, and both provide circularly polarized radiation off the orbit plane up to energies of several hundreds of keV. Measurements of the spectral brightness and the polarization components by energy dispersive powder diffraction and Compton scattering from iron in an alternating magnetic field are presented and compared with calculations based on source parameters. The beamline optics are based on use of horizontally reflecting bent Si crystals. These provide various focusing geometries, and the calculated flux of monochromatic radiation is given at energies between 30 and 175 keV. The maximum of 2×1012 photons/s is reached at 80 keV. The main experimental apparatus in the initial operation includes a high-resolution three-axis diffractometer, and a scanning-type Compton spectrometer.

High-resolution diffractometer for structural studies of polycrystalline materials

A high-resolution powder diffraction station with synchrotron radiation has been created. The diffractometer was developed on the basis of two Microcontrol precision goniometers. The latter provide the independent movement of a specimen and a detector with a minimal step of 2θ=0.001°. Using the Si(111) crystal as the analyzer, we obtained half widths of the (212), (203), and (301) reflections for α-SiO2 (0.02–0.025° 2θ). The device was tested using α-Al2O3 as the standard.