X-Ray diffraction

In order to do XRD, you need to (i) go to radiation safety training (information here) and (ii) arrange for machine-specific training.

Scheduling of the XRD equipment in Gilbert Hall is via this online scheduler at this page (as of 2013-05-01): http://www.science.oregonstate.edu/~kykynesr/

There are a few XRD systems that we use on a regular basis.

Bruker D8 Discover - Dearborn 201- Mostly used for thin films, although you can also put powders in it. Can be used to do phi scans, chi scans, rocking curve measurements, xrr, etc (these require special training because you have to use the scintillator, which isn't normally installed). Be sure to use the “snout” on the x-ray source, otherwise you end up with a broad background radiation signal.

Rigaku Ultima IV multipurpose X-ray diffraction system - Gilbert 020 - also XRR

Rigaku Miniflex Xray Diffractometer - Gilbert room # - Useful mostly for powders. The x-ray beam is too weak for much thin film work, though it may be usable if you have strong peaks (e.g. from epitaxial films).

Rigaku RAPID Diffractometer - Gilbert 020- This machine is particularly nice for doing theta/2theta thin film scans because it collects data over a wide range at once, so you can get lots of counts in a short time frame.

• Crystal Maker Home page for our lab software company. We own a 5-user license for Crystal Maker, which allows you to draw crystal structures and manipulate them, and to simulate diffraction patterns. Mac only, so you have to use lab computers.
• VESTA A free, cross-platform program (Windows, Mac, Linux) for making and visualizing crystal structures. It can also simulate powder diffraction patterns. Output is to an Igor Pro .itx file, but this is just plain text and can be imported into any plotting program.
• FindIt: A NIST database of cif files. The software lives on the PC in Wngr 481. Login via your onid or science account. Lots of search and visualization capabilities. A short intro (found on the web; no guarantees) is here: FindIt intro
• Bilbao Crystallographic Server
• Crystallography Open Database Free data base of crystal structures & cif files
• Open Access Crystallography at Portland State Open access cif files
• Powder Diffraction Files (JCPDS from ICDD). Access to this resource in the Valley Library is on the middle PC on the long portion of the the “L” shaped desk behind the “Ask Here” desk on the first floor. The machine is marked “Powder Diffraction”. Login with ONID ID and find the PDF icon on the desktop. Software is easy to use. Take a USB drive to print PDF files to it.

There is a good set of tutorials at MIT:

• Basic XRD tutorial (Speakman, MIT)
• Powder XRD data analysis tutorial(Speakman, MIT)
• Thin film XRD tutorial (Speakman, MIT)

This is useful when you have an oriented film. Say you have an 001 film and you want to figure out a and b, for example. You could do that by looking at the h0l and 0kl peaks by tilting the sample. On the Bruker system, this would be a rotation in chi. To figure out what you should set the angle to, see Cullity Appendix 1 (A1-3 has interplanar angles).

If you happen to have Mathematica handy, you can use the following notebooks (instructions included) to calculate the angles quickly:

• Interplanar Angles - Orthorhombic
• Interplanar Angles - Tetragonal

Lattice parameters can be accurately determined using a Nelson-Riley fit of your x-ray data (c vs cos²θ/sinθ, find the y intercept). You need very good data to get a reasonable parameter.

• If you're using the Bruker diffractometer, use the scintillator detector. The z alignment is both critical and tricky in this system. Zoom in as much as possible and get the laser beam as close to the middle as possible.
• If you're using the Rigaku Ultima, do a precise alignment.
• Slowly scan over a very wide range (1-2 deg/min).
• When you make a plot, the peaks should lie on a line with a slope of near zero. The y intercept is your lattice parameter.

If your film is strongly oriented, you can get the other parameters by rotating in χ. E.g. if you have a (001) film, you can get a and b by looking at (h0l) and (0kl) peaks. These lattice parameters will be determined less accurately - the signal is small and any error in c will propagate into a and b.

Can get thin film thickness by modelling fringes (XRay refractive index very close to 1, so no problems with knowing index as in optical measurements). Critical angle for Total External Reflection gives density of film (XRay refractive index in a material is slightly less than 1, so the familiar optical “TIR” becomes external refleciton). Surface roughness can be measured. Some modelling required.