Journal of Applied Crystallography

Dependence of small-angle neutron scattering contrast on the difference in thermal expansions of phases in two-phase alloys

Strunz, P. — 2009-10-03

Theoretical expressions describing small-angle neutron scattering (SANS) contrast dependence on temperature in the region where no phase-composition changes occur were derived for two-phase Ni superalloys. The theory is based on the difference in thermal expansion of the two primary phases, γ and γ′. The simulations show that the scattering contrast temperature evolution is significant enough to be considered in in situ SANS experiments with superalloys at elevated temperatures. The simulations performed show that the magnitude of the scattering contrast at room temperature is firmly connected with the particular shape of the scattering contrast temperature dependence. This fact can be used for determination of the scattering contrast without a knowledge of the compositions of the individual phases. The theoretical expressions derived for scattering contrast were proven experimentally on an Ni–Fe-base alloy, DT706. The evolution of lattice parameters of both the matrix and the precipitate phases was obtained from an in situ wide-angle neutron diffraction experiment. The theoretical scattering contrast dependence was then successfully fitted to the measured SANS integral intensity.

Management of metadata and automation for mail-in measurements with the APS 11-BM high-throughput, high-resolution synchrotron powder diffractometer

Toby, B.H. — 2009-10-15

A high-resolution and high-throughput synchrotron powder diffractometer has been automated for use with samples that are mailed in by Advanced Photon Source users. Implementation of a relational database with web interfaces for both outside users and beamline staff, which is integrated into the facility-wide proposal and safety system, allows all aspects of beamline management to be integrated. This system permits users to request kits for mounting samples, to provide sample safety information, to obtain their collected data and to provide usage information upon project completion in a quick and simple manner. Beamline staff use a separate interface to note receipt of samples, schedule and collect diffraction data, post-process and quality-check data, and dispose of samples. The design of the software and database are discussed in detail.

Study of structural defects in ZnGeP2 crystals by X-ray topography based on the Borrmann effect

Okunev, A.O. — 2009-10-30

The first study of structural defects in a ZnGeP2 semiconducting nonlinear optical crystal has been carried out by X-ray topography, based on the Borrmann effect, and the effect of anomalous transmission of X-rays on ZnGeP2 crystals has been examined. It is shown that the rosette technique of defect study under conditions of the Borrmann effect, developed earlier for elementary semiconductors, can be applied to the study and identification of defects in ZnGeP2. Features of contrast from individual edge and screw dislocations, microdefects, and coherent and semi-coherent microinclusions were considered. Defect identification was carried out by comparison of the experimental intensity contrast with simulated images of the defects.

Detection of the standing X-ray wavefield intensity inside a thin crystal using back-diffraction topography and imaging

Honnicke, M.G. — 2009-10-30

The standing X-ray wavefield into a single-crystal bulk is characterized by a combination of the diffracted–reflected h-beams and the diffracted–transmitted o-beam. For different angular positions on the total reflection region, the standing X-ray wavefield has its maximum from the region between the atomic planes (low photoelectric absorption) to the region on the atomic planes (high photoelectric absorption). Historically, the evidence for such a characteristic has come from experiments such as anomalous transmission (Borrmann effect, originally detected in Laue geometry) and fluorescent measurements with a single crystal under diffraction conditions. In the present work, such a characteristic is demonstrated by the direct measurement of the standing X-ray wavefield intensity into a 50 µm-thick single-crystal CCD detector (Si 800) set in back-diffraction geometry.

Design challenges and performance of nested neutron mirrors for microfocusing on SNAP

Ice, G.E. — 2009-10-30

Kirkpatrick–Baez (KB) neutron supermirrors can efficiently focus polychromatic neutron beams to micrometre dimensions. The ultimate size is determined mainly by the perfection of the mirrors and by the size of the beam needed to have sufficient experimental signal. Nested or Montel KB mirrors can collect ∼2.6 times more beam than standard sequential KB optics, but require good figure perfection at the edge of one mirror. This paper describes the characterization of the figure errors over the important reflective portions of the two mirrors needed for a Montel focusing pair. The measurements are placed in context with theoretical predictions and are used to predict mirror focusing performance. Strategies to improve on the focusing of this class of optics are suggested and early results from these mirrors installed on the Spallation Neutrons at Pressure (SNAP) Beamline 3 at the Spallation Neutron Source (SNS) at Oak Ridge are presented.

Small-angle neutron scattering analysis of a water-based magnetic fluid with charge stabilization: contrast variation and scattering of polarized neutrons

Avdeev, M.V. — 2009-10-30

Structure analysis of a magnetic fluid (nanoparticles of maghemite dispersed in water with charge stabilization and without surfactant) by means of small-angle neutron scattering is presented. A combination of the contrast variation technique and scattering of polarized neutrons was applied. In the first case, the scattering curves obtained for the unmagnetized fluid with variation of the heavy water content in the carrier are treated in terms of the basic functions approach. The almost homogeneous character of the nanoparticles with respect to the nuclear scattering length density makes it possible to separate information about their characteristic nuclear and magnetic radii. Polarized neutrons are then used to separate and analyze independently the nuclear and magnetic scattering contributions for the fully magnetized fluid. Both methods reveal a significant excess of the apparent nuclear size over the magnetic one, which is explained by a difference in the nonmagnetic and magnetic interactions in the system. The results indicate that from the viewpoint of magnetic interaction the studied fluid behaves under a magnetic field as a purely superparamagnetic system of independent particles. The magnetic scattering length density of the maghemite nanoparticles is found to be ∼25% less than the bulk value, which is in agreement with the data of the magnetization analysis.

Efficient calculation of a normal matrix–vector product for anisotropic full-matrix least-squares refinement of macromolecular structures

Strokopytov, B.V. — 2009-11-07

A novel algorithm is described for multiplying a normal equation matrix by an arbitrary real vector using the fast Fourier transform technique during anisotropic crystallographic refinement. The matrix–vector algorithm allows one to solve normal matrix equations using the conjugate-gradients or conjugate-directions technique without explicit calculation of a normal matrix. The anisotropic version of the algorithm has been implemented in a new version of the computer program FMLSQ. The updated program has been tested on several protein structures at high resolution. In addition, rapid methods for preconditioner and normal matrix–vector product calculations are described.

Scherrer grain-size analysis adapted to grazing-incidence scattering with area detectors

Smilgies, D.-M. — 2009-10-15

Ever since its formulation, the Scherrer formula has been the workhorse for quantifying finite size effects in X-ray scattering. Various aspects of Scherrer-type grain-size analysis are discussed with regard to the characterization of thin films with grazing-incidence scattering methods utilizing area detectors. After a brief review of the basic features of Scherrer analysis, a description of resolution-limiting factors in grazing-incidence scattering geometry is provided. As an application, the CHESS D1 beamline is characterized for typical scattering modes covering length scales from the molecular scale to the nanoscale.

The MORPHEUS protein crystallization screen

Gorrec, F. — 2009-11-07

A 96-condition initial screen for protein crystallization, called MORPHEUS, has been developed at the MRC Laboratory of Molecular Biology, Cambridge, England (MRC-LMB). The concept integrates several innovative approaches, such as chemically compatible mixes of potential ligands, new buffer systems and precipitant mixes that also act as cryoprotectants. Instead of gathering a set of crystallization conditions that have already been successful, a selection of molecules frequently observed in the Protein Data Bank (PDB) to co-crystallize with proteins has been made. These have been put together in mixes of similar chemical behaviour and structure, and combined with buffers and precipitant mixes that were also derived from PDB searches, to build the screen de novo. Observations made at the MRC-LMB and many practical aspects were also taken into account when formulating the screen. The resulting screen is easy to use, comprehensive yet small, and has already yielded a list of crystallization hits using both known and novel samples. As an indicator of success, the screen has now become one of the standard screens used routinely at the MRC-LMB when searching initial crystallization conditions for biological macromolecules.

Aspherical electron scattering factors and their parameterizations for elements from H to Xe

Zheng, J.-C. — 2009-10-03

The formalism for, and the values of, aspherical electron scattering factors using relativistic wavefunctions are presented. The parameterizations of electron form factors valid for a full range of scattering vectors from 0.0 to 6.0 Å−1 were obtained for atoms with atomic number 1 (H) to 54 (Xe) by fitting a linear combination of Gaussian functions. Eight Gaussians were needed to achieve high-quality fittings. The tabulated aspherical p- and d-orbital parameters are invaluable for probing orbital electrons and calculating high-resolution electron microscopy images and diffractions, especially those involving interfaces and defects.

The crystal structure of perdeuterated methanol monoammoniate (CD3OD·ND3) determined from neutron powder diffraction data at 4.2 and 180 K

Fortes, A.D. — 2009-10-03

The crystal structure of perdeuterated methanol monoammoniate, CD3OD·ND3, has been solved from neutron powder diffraction data collected at 4.2 and 180 K. The crystal structure is orthorhombic, space group Pbca (Z = 8), with unit-cell dimensions a = 11.02320 (7), b = 7.66074 (6), c = 7.59129 (6) Å, V = 641.053 (5) Å3 [ρcalc = 1162.782 (9) kg m−3] at 4.2 K, and a = 11.21169 (5), b = 7.74663 (4), c = 7.68077 (5) Å, V = 667.097 (4) Å3 [ρcalc = 1117.386 (7) kg m−3] at 180 K. The crystal structure was determined by ab initio methods from the powder data; atomic coordinates and anisotropic displacement parameters were subsequently refined by the Rietveld method to Rp < 3% at both temperatures. The crystal comprises a sheet-like structure in the bc crystallographic plane, consisting of strongly hydrogen bonded elements; these sheets are stacked along the a axis, and adjacent sheets are linked by what may be comparatively weak C—D...O hydrogen bonds. Within the strongly bonded sheet structure, ND3 molecules are tetrahedrally coordinated by the hydroxy moieties of the methanol molecule, accepting one hydrogen bond (O—D...N) of length ∼1.75 Å, and donating three hydrogen bonds (N—D...O) of length 2.15–2.25 Å. Two of the methyl deuterons appear to participate in weak interlayer hydrogen bonds (C—D...O) of length 2.7–2.8 Å. The hydrogen bonds are ordered at both 4.2 and 180 K. The relative volume change on warming from 4.2 to 180 K, ΔV/V, is +4.06%, which is comparable to, but more nearly isotropic (as determined from the relative change in axial lengths, e.g. Δa/a) than, that observed in deuterated methanol monohydrate.

Simulation of the powder diffraction pattern of randomly restacked Ca2Nb3O10 nanosheets

Onoda, M. — 2009-10-15

From X-ray powder diffraction pattern features, layered KCa2Nb3O10·nH2O synthesized through flocculation of delaminated Ca2Nb3O10 nanosheets with K ions appeared to be composed of randomly stacked nanosheets. Powder pattern simulation was conducted based on the matrix method using a random stacking model. When seven sheets were used as the coherent thickness, agreement in pattern fitting between the experimental and calculated intensities was satisfactory, and information about textures and atomic positions was obtained.

Basal plane bending of 6H-SiC single crystals observed by synchrotron radiation X-ray topography

Ning, L. — 2009-11-07

Basal plane bending is a structural defect in SiC single crystals caused mainly by the thermal mismatch between seed and holder, which deteriorates the quality of the wafers and blocks their applications. In this paper, basal plane bending was detected by high-resolution X-ray diffractometry (HRXRD) and transmission synchrotron white-beam X-ray topography (SWBXT). HRXRD reveals that the (0001) Si face is a concave sphere and SWBXT shows that the shapes of the Laue spots are different from that of the cross section of the synchrotron radiation beam. On the basis of a spherical curvature model for a (0001) 6H-SiC single crystal, the shapes of the Laue spots were simulated. The results are in good agreement with the experimental observations. Thus, SWBXT is an effective method for detecting basal plane bending.

In situ diffraction strain analysis of elastically deformed polycrystalline thin films, and micromechanical interpretation

Faurie, D. — 2009-10-15

In situ tensile tests have been carried out under synchrotron radiation on supported gold (Au) thin films exhibiting a pronounced crystallographic texture. The 2θ shift of X-ray diffraction lines has been recorded for different specimen orientations and several loading levels in the elastic domain. The data obtained demonstrate the large strain heterogeneities generated within the specimen because of the intergranular interactions associated with the large elastic anisotropy of Au grains. To interpret these results, the use of a multi-scale micromechanical approach is unavoidable. The theoretical background of such methods is described, and the points where exact results can be obtained and where approximations have to be introduced are highlighted. It is shown that the Vook–Witt model, for which a general formulation is provided, is the exact solution for polycrystals exhibiting a laminate microstructure, which is a significant departure from the standard thin-film microstructures. Among several standard models used in the field, the self-consistent model is the only one that reproduces the experimental data correctly. This is achieved by accounting for the actual crystallographic texture of the specimen, and assuming pancake-shaped two-point statistics for the morphological texture. A discussion of the limitations of this approach, originally developed for bulk materials, is given for the specific case of thin films.

Structural characterization of manganese-substituted nanocrystalline zinc oxide using small-angle neutron scattering and high-resolution transmission electron microscopy

Roy, B. — 2009-10-03

A series of zinc oxide (ZnO) nanoparticles, substituted with manganese di-oxide, have been synthesized through a modified ceramic route using urea as a fuel. X-ray diffraction and high-resolution transmission electron microscopy studies indicate that the sizes of the ZnO particles are of nanometer dimension. Particles remain as single phase when the doping concentration is below 15 mol%. Small-angle neutron scattering indicates fractal-like agglomerates of these nanoparticles in powder form. The size distributions of the particles have been estimated from scattering experiments as well as microscopy studies. The average particle size estimated from small-angle scattering experiments was found to be somewhat more than that obtained from X-ray diffraction or electron microscopy measurement.

Characterization of order domains in γ-TiAl by orientation microscopy based on electron backscatter diffraction

Zambaldi, C. — 2009-10-15

A new approach to resolve the slight tetragonality of L10-ordered γ-TiAl by electron backscatter diffraction (EBSD) is presented. The phase has a c/a ratio of only about 2% larger than unity. The corresponding EBSD patterns therefore exhibit cubic pseudosymmetry. As a consequence, different order variants cannot be easily distinguished on the basis of their EBSD patterns. Automated orientation mapping results in frequent misindexing. In the past, either this problem was overcome by identifying order domains by relatively laborious transmission electron microscopy, or the order domain structure was ignored altogether by using a generic face-centered cubic structure to solve for the crystal orientations, accepting a significant loss of microstructural information. The presented approach is based on the detection of the minor tetragonal distortion of the diffraction patterns by an accurate measurement of backscatter Kikuchi band positions. To this end an accurate pattern center calibration together with high-accuracy parameters for pattern acquisition and indexing are required. Together with a modified indexing algorithm, the order domains in a lamellar microstructure of Ti–45.9Al–8Nb (at%) could be reliably identified. The occurrence of superlattice reflections in the Kikuchi patterns was used to validate the technique. The developed method was successfully applied to create a crystal orientation map of Ti–45.9Al–8Nb (at%) with a fully resolved domain microstructure.

Measurement of stress factors and residual stress of a film by in situ X-ray diffraction during four-point bending

Ortolani, M. — 2009-10-15

A method is proposed for the simultaneous measurement of the stress factors and residual stress state of a film by in-situ X-ray diffraction during four-point bending. The externally applied load acts as an additional degree of freedom in the stress–strain relation, which allows the problem to be solved for both residual stress and elastic moduli without assuming any grain interaction model. The procedure was tested on a galvanic nickel deposit and the predictions of existing grain interaction models were compared with the experimental results.

Towards extracting the charge density from normal-resolution data

Dittrich, B. — 2009-10-03

The limiting factor for charge-density studies is crystal quality. Although area detection and low temperatures enable redundant data collection, only compounds that form well diffracting single crystals without disorder are amenable to these studies. If thermal motion and electron density ρ(r) were de-convoluted, multipole parameters could also be refined with lower-resolution data, such as those commonly collected for macromolecules. Using the invariom database for first refining conventional parameters (x, y, z and atomic displacement parameters), de-convolution can be achieved. In a subsequent least-squares refinement of multipole parameters only, information on the charge density becomes accessible also for data not fulfilling charge-density requirements. A critical aspect of this procedure is the missing information on the correlation between refined and non-refined parameters. This correlation is investigated in detail by comparing a full multipole refinement on high-resolution and a blocked refinement on `normal-resolution' data sets of ciprofloxacin hexahydrate. Topological properties and dipole moments are shown to be in excellent agreement for the two refinements. A `normal-resolution' data set of ciprofloxacin hydrochloride 1.4-hydrate is also evaluated in this manner.

Structural studies on carbon nanotube fibres by synchrotron radiation microdiffraction and microfluorescence

Davies, R.J. — 2009-10-15

This study reports on the characterization of a carbon nanotube fibre using synchrotron radiation microbeam small- and wide-angle X-ray scattering in combination with microfluorescence. The fibre, spun directly from a chemical vapour deposition reaction zone, is imaged in terms of microstructural heterogeneities. The results reveal a fibre consisting of highly oriented nanotube bundles and unoriented carbonaceous material. Within the oriented component there is a variable orientation distribution and evidence of differences in nanotube packing. Single catalyst crystallites can be located within the fibre from their wide-angle X-ray scattering signal, and the particulate distribution imaged using X-ray microfluorescence. Whilst this study only constitutes a preliminary analysis, it demonstrates the application of existing fibre characterization methods to new materials. It also highlights the potential of synchrotron radiation micro- and nanobeam small- and wide-angle X-ray scattering and microfluorescence for the study of fibres of a few µm diameter.

Grating-based holographic X-ray diffraction: theory and application to confined fluids

Nygård, K. — 2009-10-30

A grating-based holographic X-ray diffraction technique has been developed for reconstructing density profiles of nano-scale fluids confined in channel arrays. Within this approach, the reference wave is due to diffraction from the fabricated channel array, whereas the object wave is generated by the confinement-induced ordering of the fluid. The ensemble-averaged density profile of the fluid across the confining channel, which constitutes a weak phase object, is then determined in a model-independent manner from the interference between the reference and object waves by direct Fourier inversion. The validity of the linear holographic approach and its connection to the autocorrelation function, the inclusion of channel tapering, and volume-diffraction effects are discussed in detail.

Energy-dispersive Laue diffraction by means of a frame-store pnCCD

Send, S. — 2009-10-30

A frame-store pn-junction CCD detector was applied to the energy-dispersive X-ray Laue diffraction study of a γ-LiAlO2 crystal with white synchrotron radiation. Exploiting the simultaneous spatial and energy resolution of the detector the crystallographic unit cell of γ-LiAlO2 could be determined without any a priori information about the sample. The potential for application in X-ray structure analysis is tested by comparing experimental structure factors taken under a single exposure with those calculated from the known crystal structure. After correcting the measured spot intensities by angular and energy-dependent parameters, the agreement between experimental and theoretical kinematical structure factors is better than 10%.

Comparison of X-ray and electron backscatter diffraction textures for back-annealed Al–Mg alloys

Engler, O. — 2009-11-07

The potential of electron backscatter diffraction (EBSD) to determine integral macrotexture data is explored by comparing EBSD-derived textures with standard X-ray texture results. The comparison is performed for an Al–Mg alloy AA 5005 in the cold-rolled and various back-annealed states in order to analyse the impact of the microstructural state on the quality of EBSD-based macrotextures. The number of EBSD single orientation measurements necessary to represent a texture adequately is determined by way of exploring the convergence of the statistical parameter ρ, which represents the relative mean square deviation between the EBSD and X-ray-based textures. The effect of EBSD filtering tools and the impact of sampling step size on the statistical significance of EBSD data are investigated. Several means to reduce the number of data points without compromising the accuracy of the texture results as an input for subsequent texture simulation are addressed.

Depth-dependent local structures in thin films unraveled by grazing-incidence X-ray absorption spectroscopy

Souza-Neto, N.M. — 2009-11-17

A method of using X-ray absorption spectroscopy together with resolved grazing-incidence geometry for depth profiling of atomic, electronic or chemical local structures in thin films is presented. The quantitative deconvolution of thickness-dependent spectral features is performed by fully considering both scattering and absorption formalisms. Surface oxidation and local structural depth profiles in nanometric FePt films are determined, exemplifying the application of the method.

Development of a shutterless continuous rotation method using an X-ray CMOS detector for protein crystallography

Hasegawa, K. — 2009-11-17

A new shutterless continuous rotation method using an X-ray complementary metal-oxide semiconductor (CMOS) detector has been developed for high-speed, precise data collection in protein crystallography. The principle of operation and the basic performance of the X-ray CMOS detector (Hamamatsu Photonics KK C10158DK) have been shown to be appropriate to the shutterless continuous rotation method. The data quality of the continuous rotation method is comparable to that of the conventional oscillation method using a CCD detector and, furthermore, the combination with fine ϕ slicing improves the data accuracy without increasing the data-collection time. The new method is more sensitive to diffraction intensity because of the narrow dynamic range of the CMOS detector. However, the strong diffraction spots were found to be precisely measured by recording them on successive multiple images by selecting an adequate rotation step. The new method has been used to successfully determine three protein structures by multi- and single-wavelength anomalous diffraction phasing and has thereby been proved applicable in protein crystallography. The apparatus and method may become a powerful tool at synchrotron protein crystallography beamlines with important potential across a wide range of X-ray wavelengths.

Neutron powder diffraction in materials with incoherent scattering: an illustration of Rietveld refinement quality from nondeuterated gypsum

Henry, P.F. — 2009-11-17

The power of the state-of-the-art neutron powder diffractometer suite at the Institut Laue–Langevin for investigating the structure of nondeuterated materials is presented using gypsum, CaSO4·2H2O, as a reference material. It is shown that flexible modern neutron powder diffraction instruments at reactor-based sources can yield data with sufficient counting statistics above the incoherent scattering contribution to perform unconstrained refinements in relatively short time periods (from minutes to a few hours, depending on the sample size and the instrument choice), without the requirement for significant changes to the standard operational modes of the instruments. The results are critically compared with previous literature from single-crystal and powder X-ray and neutron measurements on deuterated and nondeuterated gypsum.

PeckCryst: a program for structure determination from powder diffraction data using a particle swarm optimization algorithm

Feng, Z.J. — 2009-10-03

PeckCryst has been developed for the solution of molecular crystal structures from powder diffraction data using a particle swarm optimization (PSO) algorithm. In order to speed up the calculation process, a modified Bragg R factor is used as the evaluation function for the PSO algorithm. The effectiveness of the program has been tested by solving four known structures from powder diffraction data. A Python script is also provided for convenient repetitive running of PeckCryst. The distributed PeckCryst program is freely available from the authors upon request, and runs on Linux and Windows (32- and 64-bit) platforms.

VIBRATE! A program to compute irreducible representations for atomic vibrations in crystals

Glazer, A.M. — 2009-10-15

VIBRATE! is a computer program that uses group theory to carry out factor group analysis of a crystal structure. The symmetry species of the normal modes of vibration are derived, together with information relating to the symmetry-adapted vectors. The program is simple to use, relying on input mainly from a crystallographic information file. The output is presented in a form that should be familiar not only to crystallographers but also to others such as chemical spectroscopists.

EXPO2009: structure solution by powder data in direct and reciprocal space

Altomare, A. — 2009-11-17

The program EXPO2009 is the evolution of EXPO2004 [Altomare, Caliandro, Camalli, Cuocci, Giacovazzo, Moliterni & Rizzi (2004). J. Appl. Cryst. 37, 1025–1028]. EXPO2009 performs all the steps of ab initio structure solution by powder data: indexing, space-group determination, estimation of the reflection integrated intensities, structure solution by direct/Patterson methods and/or by a direct-space/hybrid approach, and model refinement by the Rietveld technique. New procedures have been introduced in EXPO2009 for enhancing the structure solution process, particularly in the case of low-resolution data and/or organic compounds, when traditional approaches like direct methods may fail. The EXPO2009 graphical interface has been optimized and made very user friendly.

A method for the indexation of back-reflection Laue X-ray photographs

Sidokhine, F.A. — 2009-10-03

This article presents a variation of the traditional method for indexing and determining the crystal orientation from Laue back-photographs with a demonstration of its applicability to low-quality diffraction patterns from imperfect metallic single crystals.

Test tube for obtaining crystals

Cabric, B. — 2009-10-03

A design for an air-cooled test tube with a series of sockets and plugs (`crystallization socket-outlet adaptor') installed in a laboratory furnace is presented. The setup allows easy regulation and simultaneous crystallization tests of a series of different crystallization parameters and substances, enabling fast studies of single-crystal growth.

A robotic arm as a simple sample changer for a diffractometer with very low component costs

Lian, D. — 2009-10-15

Inexpensive model robots are a viable option for automation of simple, repetitive tasks and can be solutions when space restriction and funding are issues, both factors that may eliminate more advanced robots from consideration. A simple-to-program, inexpensive robotic arm has been integrated in a sample changer for room-temperature experiments on a neutron powder diffractometer. In spite of the limited precision inherent in a model, servo-controlled robot, a very reproducible overall system can be made. Simple `tricks' such as incorporating self-centering mechanisms, e.g. mechanically self-centering designs and magnets, can produce central forces that eliminate the need for high precision from the robot arm.