ID22 is a versatile instrument and can be exploited for a wide range of powder diffraction measurements including:

  • Structural studies: the solving and refining of crystal structures, investigation of structure in crystalline, defective and non-crystalline materials via atomic pair distribution function (PDF) analysis,
  • Dynamic and in-situ studies: observation of structures or materials evolving with temperature, time, voltage, etc. during phase changes, gas adsorption, solid-state chemistry, electrochemistry, etc.;
  • Anomalous scattering: distinguishing between neighbouring elements in a material. We can access the K or L edge (sometimes both) for all elements above Mn (6.5 keV) in the Periodic Table;
  • High throughput studies: involving many samples, synthesised with different compositions, or under varied preparation conditions, etc. A sample-changing robot allows automated screening of up to 75 capillaries in succession, with temperature control in the range 80 K - 950+°C;
  • Quantitative analysis: owing to the high angular resolution, diffraction patterns from complex mixtures with many contributing phases can be analysed; moreover, with the excellent statistical quality and low background, the detection of phases present in very low proportions is possible;
  • Microstructure: detailed analysis of peak shapes yields information about the microstructure of a material. Since the instrumental contribution is small, the observed peak shapes are dominated by sample effects;
  • Residual strain: Measurements of residual strain, either by the sin2ψ technique, or by mapping peak positions from within the bulk and surface of a sample, defining a gauge volume with slits and the angular acceptance of the analyser crystal.

Structural studies

Crystalline materials – classic crystallography

The combination of narrow peaks, accurate peak positions and intensities is essential for crystallographic studies using powder diffraction, which are necessary when a single crystal of a material is unavailable. The effective 2θ resolution of a diffractometer equipped with an analyser crystal, (Δθ/θ), is only weakly dependent on the wavelength (improving marginally at longer wavelengths) so that the wavelength chosen for an experiment is determined primarily by questions of absorption and unit cell size. Thus powder patterns for structural studies are usually measured on ID22 using X-rays in the range 0.3 – 1.5 Å, (40 – 8 keV) depending on the nature of the sample. Inorganic specimens containing metals or heavy elements, a large part of the work on ID22, are measured in the range 0.3 – 0.4 Å with capillary diameters adopted to keep µr < 1.5, when a simple correction for absorption is possible (Optimum signal in the detector is obtained with µr = 1). Even materials with quite large unit cells can be measured at this wavelength as good quality powder data can be obtained, with care, down to angles as low as 0.6° 2θ. For samples with very large unit cells, such as proteins, (where the former powder diffraction beamline ID31 registered a number of successful studies), wavelengths in the range of 1.25 – 1.5 Å may be chosen, to move the peaks to diffraction angles further away from the beam stop and the low-angle air scatter from the main beam.

► Examples:

  1. Inorganic and organic materials
  1. Framework structures:
  1. Proteins:

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Poorly crystalline materials – atomic pair distribution function (PDF) method

For materials that are not good crystalline powders a method that is proving increasingly important is derivation of the atomic pair distribution function (PDF). This technique can be applied to structures that lack the translational periodicity of a crystal, and thus used to investigate poorly-crystalline or disordered materials. The method has been comprehensively reviewed [1]. A key requirement is that data should be measured to high values of Q, (4πsin(θ)/λ), ideally up to or beyond 30 Å-1, to give high real-space resolution. Thus the measurements need to be made with short-wavelength radiation. The atomic PDF is a measure of the number of atoms in a shell of radius r about a reference atom. Peaks represent the characteristic distances between pairs of atoms in the structure. Two methods of measuring the X-ray data for a PDF analysis are favoured.

The first uses an instrument like ID22 with analyser crystals and a wavelength of around 0.3 – 0.4 Å, scanning to beyond 90° 2θ, and recording the weakly-scattering high-angle data repeatedly to improve the statistical quality. Data collection takes a number of hours, but the analyser crystals help suppress the background due to Compton scattering, fluorescence, etc.

An alternative approach uses a very short wavelength of ≈0.1 Å and an area detector to record the full diffraction pattern in a single shot. The angular resolution of the pattern is lower, and the background is less effectively suppressed, however hours of data collection are reduced to just minutes, which is obviously an advantage if following a system evolving during the course of the experiment. This "fast-PDF" approach will be developed as an option in the ID22 beamline in the course of 2014/15. A comparison of the two methods (image plate at APS vs. scanning with an analyser crystal at ID31) with a series of icosahedral alloys [2] suggested that they give similar results, but with the analyser crystal measurements yielding somewhat clearer information in the high r region.

► Examples:

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Dynamic / in-situ measurements

One of the major general strengths of powder diffraction is the ability to carry out measurements under a wide range of environmental conditions. Dynamic measurements have included the exploration of phase diagrams with temperature, the kinetics of in-situ chemical reactions such as dehydration of samples on heating or hydrothermal synthesis, the kinetics of physical changes such as the growth of nanoparticles by high-temperature annealing, and following the changes in the crystalline components in electrochemical cells with charging and discharging, etc.

► Examples

 

Microstructural studies

As mentioned above, the very high intrinsic resolution of ID22 means that peak shapes are dominated by sample effects, allowing the data to be used to characterise microstructural effects in a material.

Organisms can control the morphology, orientation, size and hence properties of crystalline blocks in biogenic crystals (e.g. calcite in shells) by means of organic molecules involved in the biomineralization process, though detailed understanding of the mechanisms are lacking. Proteins are implicated, and it is believed that amino acids from the protein chain can adhere to specific planes of the mineral phase and impede growth in the perpendicular direction.

► Examples

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Residual strain 

Residual strain in engineered components can be investigated using the traditional sin2ψ technique, whereby the position of a diffraction peak is monitored in reflection geometry at different sample orientations (ψ). Alternatively, with the availability of X-rays up to ~80 keV on ID22, a gauge volume in the sample can be defined by means of the incident and diffracted beams. A map of residual strain is built up by translating the sample parallel and perpendicular to the beam and measuring the 2θ position of a peak for the different sample positions. High spatial resolution is available by working with narrow beams, though the gauge volume is lengthened by the low values of 2θ that arise from the use of hard X-rays. Moreover, there is little point in working with a beam that is of comparable size to the intrinsic grain size of the sample, because a poor, spotty powder pattern results. Generally on ID22, beams down to 50 µm (horizontal or vertical) are acceptable, though because of the lack of focussing, small beams represent a large loss of intensity.

In the future, a transfocator will be available for focusing and improve ID22 strain-scanning capabilities.

There are several advantages in using synchrotron X-rays and a high-resolution powder diffractometer equipped with an analyser crystal,

  1. The intrinsically narrow peak widths give enhanced sensitivity to shifts in peak positions;
  2. The peak widths are independent of ψ, unlike traditional Bragg-Brentano geometry, where loss of parafocusing leads to peak broadening;
  3. The wavelength tunability gives versatility. Soft X-rays can be selected for surface sensitivity. The depth of penetration can be varied by changing wavelength, even exploiting absorption edges if appropriate. Hard X-rays (lambda ≤ 0.3 Å) allow transmission measurements and strain mapping.
  4. The use of an analyser crystal leads to accurate measurements free from the effects of surface shape or roughness, and confers immunity to aberrations that lead to undesired shifts in peak positions with conventional instruments, e.g. resulting from movement of the sample from the diffractometer axis, or surface effects when the gauge volume is only partially immersed in the sample. Thus components with complex shapes are readily investigated.

► Examples

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References:

[1] Beyond crystallography: the study of disorder, nanocrystallinity and crystallographically challenged materials with pair distribution functions. S. J. L. Billinge, and M. Kanatzidis, Chem. Commun., 749 (2004).

[2] PDF from X-ray powder diffraction for nanometer-scale atomic structure analysis of quasicrystalline alloys.  S. Brühne, E. Uhrig, K.D. Luther, W. Assmus, M. Brunelli, A. S. Masadeh, and S. J. L. Billinge, Z. Kristallogr., 220, 962–967 (2005).