Other diffraction techniques

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Student authors: Katie Kidder 2018

How electron/neutron and SAXS/WAXS different

Electron Diffraction, a beam of electrons is shot at a thin layer of a material, on the order of 100 nm. Is the only reliable method for obtaining crystal structure information on crystals that are on the order of 0.01 nm in diameter. Single crystal electron diffraction produces Kikuchi lines which can be used to align crystals for TEM.

Example Some indexed Kikuchi lines for a [110]bcc diffraction pattern when the crystal is oriented precisely along the [110] zone axis

Neutron Diffraction, neutrons are generated by a nuclear reactor and then sent through a monochromator before being scattered through the crystal. Neutrons are scattered differently by atoms than x-rays are. For example hydrogen does not diffract x-rays well, whereas neutrons are very strongly scattered by hydrogen

SAXS, small angle x-ray scattering. WAXS, wide angle x-ray scattering. SAXS is done with an angle 2 θ less than 5 degrees and is most often used to examine features greater than 50 nm in size. WAXS is done with an angle of greater than 5 degrees. WAXS is generally used to examine polymers and can be used to detect features in the approximately 10 nm or less area.

How to interpret the data it generates

These techniques are presented similarly to XRD in that they provide verification of crystal structure. Two compounds with the same pattern have the same crystal structure. These techniques are frequently used in conjunction with other data to support that a synthesis has been successful by comparing with a reference pattern, or by indicating that a change in the pattern has occured over the course of the synthesis

Another use of these diffraction methods is crystal structure determination. This can be done due to the fact that a peak at each wavelength or angle (for electron/neutron or x-rays) corresponds to a certain plane in the crystal structure. If there is a peak in the spot that corresponds to the miller index, then there is something in the sample in that plane that diffracted the (x-ray/electrons/neutrons). Based on these a crystal structure can be determined, most often by comparing to a structure library.

Caveats/Errors

Electron Diffraction: Secondary Diffraction (ie the electron is scattered by multiple points in the crystal lattice) can lead to extra reflections and unreliable! beam intensities.
1. Can only be used on very small quantities of sample
Neutron Diffraction
1. Incredibly Expensive, and requires one cubic millimeter of sample
2. Beams of neutrons are weak
SAXS/WAXS
1. Same problems as XRD since they still utilize x-rays (phase problem- cannot measure the electric field, only the intensity of the x-ray but the Electric field also depends on the phase).

Broad peaks implies an amorphous crystal structure.

https://www.chem.uci.edu/~lawm/263%204.pdf

Good literature examples

This paper was looking at hydrogen content in an MX-ene. Since hydrogen scatters neutrons so strongly over a large range of wavelengths, it becomes part of the background. This paper looked at the difference between a sample with no hydrogen and a sample with a know combination of hydrogen, and then looked at a linear combination of the two in order to determine the amount of hydrogen in an unknown sample. Since all the other peaks match up, the presence of hydrogen is the only difference in this crystal structure.1

Figure 1

Figure 2

This paper was looking at the digestion of milk in the absence (Figure 1) and the presence (Figure 2) of bile compounds, and what nanostructures formed over the course of the digestion process. This paper used SAXS to look at the digestion over time. Two things can be interpreted from the figures. First it is clear that the crystal structure of the milk changes over the course of the digestion due to the fact that peaks both appear and disappear over time and with change in pH. Secondly it can be determined what the crystal structure at each point is by assigning the peaks to their corresponding Miller indices and then comparing which crystal structure those assignments belong to. The structures are shown alongside Figure 1. 2

Bibliography

(1) Muckley, E. S.; Naguib, M.; Wang, H.-W.; Vlcek, L.; Osti, N. C.; Sacci, R. L.; Sang, X.; Unocic, R. R.; Xie, Y.; Tyagi, M.; Mamontov, E.; Page, K. L.; Kent, P. R. C.; Nanda, J.; Ivanov, I. N. ACS Nano 2017, 11, 11118–11126.

(2) Salentinig, S.; Phan, S.; Khan, J.; Hawley, A.; Boyd, B. J. ACS Nano 2013, 7, 10904–10911.