Modern Diffractometers Research Articles

Diffraction methods in the characterization of new mineral species

Data from publications describing new minerals in the periods 2001–2005 and 2016–2020 have been analysed to evaluate the contributions of diffraction methods to the new mineral characterisations. The two periods were chosen to be on either side of a sudden increase in new mineral descriptions from ∼50 to 120 per year and coincidentally correspond to the changeover from the majority (60%) of the new descriptions using photographic film cameras to record the powder diffraction patterns of the minerals, to using modern diffractometers. During both periods, diffraction data obtained using laboratory-based single-crystal diffractometers accounted for almost three-quarters of the crystallographic studies. Modern diffractometers can now routinely be used to collect high-quality data on crystals down to a few μm in size, once the province of synchrotrons. Large facility-based diffraction centres like synchrotrons and scientific nuclear reactors account for less than 10% of the new mineral publications. The review focuses on diffraction methods for the characterisation of fine-grained minerals, with sub-μm sized crystals. Transmission electron microscopy/electron diffraction methods are important for these cases, with an exponential increase in 3D ED methods during the last 15 years and increased usage of electron back-scattered diffraction methods for micro-inclusion minerals, especially in meteorites.

The conformational preference of s-cis amides. Ramachandran plots.

This is really just a postscript to the previous post. There I showed how a search of the (small molecule) crystal database revealed the s-cis conformation about the N-C amide bond (the one with partial double bond character that prevents rotation) and how this conformation means that a C-H approaches quite closely to an adjacent oxygen. It is a tiny step from that search to a related, and very famous one named after Ramachandran[1]. Indeed this search, and the contour map used to display the results, really put crystal databases on the map so to speak. The search above defines two torsion angles about the central sp3-hybridized carbon atom (the one that makes amino acids chiral if that carbon does not carry a second hydrogen atom, i.e. glycine). The two angles are called Ψ and Φ. Such searches have been done countless times; but here I subject it to my usual constraints (R < 0.05, no disorder, no errors, T ≤ 175K and acyclic bonds along the backbone so as not to constrain it). The red-spot (> 15 instances) occurs at angles of Ψ and Φ of ~ +150 and -70°. If the temperature constraint is switched off (below), a distinctly different plot emerges with the red-spot Ψ reduced to -25°. If the temperature constraint is replaced by the stipulation that the structure must be published post 2000, the first plot is largely recovered, and we may conclude that the two plots differ only because of the use of more modern diffractometers, where low temperature measurement is routine. If may also mean that more difficult structures previously inaccessible to older instruments tend to dominate the more modern plot. I also show a plot where the central carbon is forced to carry two H-C bonds (glycine derivatives). As is well-known, the absence of the steric bulk on that atom does induce a different backbone conformation (and hence a different location for the red spot). And so, with this thread I have moved from discussing quite tiny molecules into the arena of much vaster biomolecules. But the basic principles are the same for both. References G. Ramachandran, C. Ramakrishnan, and V. Sasisekharan, "Stereochemistry of polypeptide chain configurations", Journal of Molecular Biology, vol. 7, pp. 95-99, 1963. http://dx.doi.org/10.1016/S0022-2836(63)80023-6

The technique of X-ray diffraction investigation of crystal aggregates

A number of experimental techniques of the X-ray diffraction investigation of crystal aggregates is presented with employment of modern diffractometers equipped with 2D CCD detectors. Fundamentally, these techniques are based on taking a series of Debye-Scherrer photographs. The measurement accuracy of the interplanar distances and unit cell parameters using internal and external standards is discussed.

Generalization of the classical xyz-polarization analysis technique to out-of-plane and inelastic scattering

The technique of longitudinal ("xyz") polarization analysis has been used successfully for many years to study disordered magnetic materials in thermal and cold neutron diffraction experiments. The technique allows the simultaneous and unambiguous separation of the nuclear, magnetic, and nuclear spin-incoherent contributions to the scattering. The technical advances seen in recent years, such as the availability of polarized (3)He analyzer cells to cover a large detector solid angle, the ability to detect out-of-plane scattering in a multi-detector, and a significant increase of the usable beam divergence, call for a generalization of the method. A general treatment of the formalism for carrying out neutron polarization analysis will be given in this paper, which describes a possible method of usage at a future, modern diffractometer or inelastic spectrometer with large area multi-detector coverage.

The testing of modern diffractometers for type approval

The testing of modern diffractometers for type approval

Use of linear scans to obtain |Fhkl|2and the integrated intensity

An alternative method for determining the structure factor of a Bragg reflection is described. For broad reflections, the structure factor can be obtained from linear scans through the reflection in reciprocal space. The best known geometry corrections,i.e.Lorentz, polarization and area (volume) factors, are properly taken into account. In modern diffractometers, scans in reciprocal space are standard operations controlled by the diffractometer software program. The necessary corrections and diffractometer slit settings for the use of the new method are described.

Instrumental aspects in X-ray diffraction on polycrystalline materials

The purpose of this paper is to give a rapid overview of the recent developments in the field of X-ray diffraction on polycrystalline materials from the viewpoint of the instruments. After a brief historical report, the main types of laboratory diffractometers are presented. At the end of the twentieth century the apparition of position sensitive detectors and artificial crystal monochromators have induced the conception of new diffractometer often based on old geometrical arrangements. Those modern diffractometers are described with respect to the more conventional ones. Among the experimental parameters which can characterize a given diffractometer, the instrumental resolution function and the acquisition time of the pattern are of primary importance. The different apparatus are compared with respect to those two parameters.

Structural Reinvestigation of Ammonium Hypophosphite: Was Dihydrogen Bonding Observed Long Ago?

AbstractFor Abstract see ChemInform Abstract in Full Text.

Structural Reinvestigation of Ammonium Hypophosphite: Was Dihydrogen Bonding Observed Long Ago?

The past decade has seen the explosive emergence of "dihydrogen bonds", interactions between the electrons of M-H sigma-bonds, where M is less electronegative than H (M = Al, B, Ga, Ir, Mo, Mn, Os, Re, Ru, W) and traditional proton donors. But 70 years ago, such an interaction was proposed by Zachariasen and Mooney [J. Chem. Phys. 1934, 2, 34-37] on the basis of their single-crystal X-ray diffraction structure (heavy atoms positions only) of ammonium hypophosphite (NH(4)H(2)PO(2)). We redetermined this structure at high resolution with a focus on the hydrogen atoms, using a modern diffractometer equipped with a CCD detector. Molecular orbital calculations were performed to investigate the charge density and the bond polarity of the P-H bonds and to assess their potential for participation in dihydrogen bonds. Neither the theory nor the X-ray structure supports the original claim of H...H interactions in this salt.

Geometry-independent intensity correction factors for grazing-incidence diffraction

A geometry-independent approach to a unified description of intensity corrections factors for the two most common surface scattering geometries is presented. Simple analytical formulas relate the representations of the Lorentz factor, polarization factor, area factor, and rod interception factor in a surface reference frame to the actual diffractometer circles. An analytical formula for the area correction for very large exit angles, as can be achieved on modern diffractometers, is derived. A test data set shows that the derived formulas provide a good description of the scattering intensity up to the edge of the Ewald sphere.

Influence of crystal size of Yellow 13-type pigments on optical and rheological properties of lithographic inks

Use of a modern diffractometer and detailed knowledge of the crystal lattice structure have improved ability to size crystals of CI Pigment Yellow 13-type pigments. This has enabled confirmation and refinement of correlations observed in earlier work between crystal sizes and optical and rheological properties of commercially available pigments in lithographic inks. The correlations indicate that the dispersion procedures used have produced inks in which the pigments exist substantially as individually dispersed crystals. The correlations also indicate that the crystals of the commercially available pigments are effectively perfect. An alternative explanation on the basis that the various pigments have similar degrees of aggregation and imperfection of crystals is also possible, but improbable due to the widely different compositions and sources of the pigments. Whether or not abietyl resin added to improve dispersibility of pigment remains on the crystal surfaces in inks has not been resolved.

The optimum standard specimen for X-ray diffraction line-profile analysis

A perfect general purpose standard specimen for high accuracy line-profile analysis is shown to be an illusion. Balancing the partly contradictory requirements, an optimum standard specimen for a parafocusing diffractometer is developed. To obtain the optimum standard specimen, a 5–10 μm particle size fraction is taken from the NIST certified Si powder SRM640a, about 1.5 mg/cm2 of this powder is uniformly deposited on a (510) oriented Si single-crystal wafer and the assembly is heat treated for 2 h at 1273 K to remove lattice imperfections. All procedures necessary are precisely given, easily applicable, and reproducing. For the present standard specimens, the random errors due to crystal statistics are quantified and shown to be acceptable for spinning specimens; the systematic errors due to residual size and transparency broadening are determined semi-empirically and can be eliminated, if desired. Thus the proposed optimum standard specimen allows the determination of instrumental line profiles free from systematic errors and with random errors in the line width of the order of 0.001 °2θ, allowing a full use of the capacities of modern diffractometers and data evaluation procedures.

Diffraction by Diffusely Scattering Materials of High Transparency

Even with well aligned modern diffractometers, the wide-range diffraction patterns of diffusely scattering materials are distorted owing to factors associated with finite specimen length, strong Compton scattering, high X-ray transparency causing asymmetric broadening and displacement of interference maxima towards smaller angles, and possibly small-angle scattering. The small-angle scattering can extend far enough to overlap the first interference maximum and the peak displacement can lead to significant d-spacing errors. Simple methods of correcting for these distortions are presented. These corrections are especially important in situations where overlapping peaks are present.

Evaluation of Reference X-ray Diffraction Patterns in the ICDD Powder Diffraction File

AbstractProcedures and tools for evaluation of reference x-ray powder patterns in the JCPDSICDD Powder Diffraction File are illustrated by a review of air-stable binary oxides. The reference patterns are evaluated using an available microcomputer version of the NBS*A1DS83 editorial program and PDF patterns retrieved directly from the CD-ROM in the program's input format. The patterns are compared to calculated and experimental diffractograms. The majority of the oxide patterns have been found to be in good agreement with the calculated and observed diffractograms, but are often missing some weak reflections routinely observed with a modern diffractometer. These weak reflections are added to the PDF pattern. For the remainder of the phases, patterns are redetermined.

An X-ray analysis of stacking disorder in kaolinite by fourth moment

Mathematically, it is well established that besides the Fourier coefficients, the natural characteristics of any distribution are its moments. The use of the second moment, called variance, as a measure of line broadening has already gained wide popularity because of its several merits relative to other empirically devised parameters. In recent decades, the fourth moment has been recognized as an important parameter; its theoretical significance has been discussed by Mitra (1964), who developed the moment range function for the estimation of crystallite size and distortion in cold-worked metal. However, owing to the requirements of high degree of accuracy of intensity measurement coupled with cumbersome calculations, this parameter did not find much general application. Modern diffractometers and sophisticated computers have helped ease the problem and in recent years several workers (Kagan & Snovidov, 1965; Kulshreshtha et al., 1971; Mukherjee, 1981) have successfully used this parameter for defect analysis in various non-ideal forms of materials.

Evaluation of the Energy Dispersive Detector as a Detector-Filter System for the X-Ray Diffractometer

AbstractA comparison of the relative merits of the energy dispersive derector-pulse height analyzer, scintillation detector-graphite monochromator, and proportional detector-pulse height analyzer combinations.Typical energy dispersive detectors are not configured for maximum efficiency on the diffractometer. Being only on the order of 3 mm diameter, a good deal of the available information is not collected by the detector. This is especially true with the Wide optics found in modern diffractometers. The energy dispersive detector incorporated into this system is optimized for the x-ray diffractometer. Its detection area is a 1.25 X 0.25 inch rectangle. The resolution is only sufficient to remove the Kβ portion of the spectrum.Conventional diffractometer techniques incorporate either a scintillation detector-crystal monochromator, or a proportional detector-pulse height analyser combination. The question posed is “what are the advantages in signal to noise ratio and pulse height distribution of the energy dispersive-pulse height analyzer over the more conventional arrangements.”

Effect of Soller Slits on X-Ray Intensity in a Modern Diffractometer

The properties of Soller slits in a modern diffractometer are discussed in relation to the effect they have on the x-ray intensity distribution within the plane that contains the line focus and diffractometer axis. On the basis of an idealized Soller slit collimator, equations are derived for computing relative x-ray intensity in terms of the distance from the edge of the specimen. An independent mathematical derivation substantiates these equations and also shows the effect of varying the number of slits while keeping other properties of the collimator unchanged. When the equations are used to calculate the over-all intensity profiles of two diffractometers at the specimen position, the results are found to be in reasonable agreement with photographically determined profiles. Similar equations are used to explain the effect of the receiving Soller slits on the diffracted rays.

Twenty Years of Progress in X-Ray Diffraction Techniques

AbstractAlthough no revolutionary advance has been achieved in the last two decades, X-ray diffraction is not to be considered as a quiescent field of physics. Actually many improvements, in theory as well as in experiment, slight by themselves but very numerous, have considerably increased the efficiency of techniques such as the determination of crystal structures, the analysis of crystalline phases, and the applications of X-rays to various problems of the physics of solids. Only the two last points will be dealt with here:1.Crystalline phase analysis. The development of a satisfactory atlas of powder patterns has been too slow, and the data are not yet complete and precise enough to permit a rational utilization of the modern diffractometers. A very interesting new approach is the systematic indexing of the powder patterns which would be possible with computers. In the near future, anyone should be able to analyze a powder at any temperature as an easy routine experiment.2.The study of lattice defects. X-ray techniques are now in competition with electron microscopy, the development of which has been very successful in recent years. Now we have a better understanding of the possibilities of both techniques. X-rays give better results to determine the statistics of an extended disorder even if it is slight (e.g., degrees of order in a solid solution), and the microscope is more powerful for the detection of large but rare defects (e.g., dislocations).