Orient Express Tutorial

Laue diffraction is a method to find the crystallographic direction of single crystals. We shine the single crystal with white x-ray (having a continuous range of wavelength), and record the diffracted pattern on a x-ray film or image plate. Analyzing the pattern, we learn about the crystal structure. Sometimes we need to compare the observed pattern with a simulated pattern to identify a crystallographic plane. Here is how we can simulate the Laue pattern of a single crystal. This tutorial is meant for the first-time users.

• Download OrientExpress here and unzip in a Windows computer. Click OrientExpress.exe to launch the program. Below is the Main Window you will see once you launch OrientExpress.

• To simulate Laue pattern of a single crystal, we need to insert the required parameter in OrientExpess. We can choose to insert the parameter manually. Go to menu bar Data » Keyboard. You will see following window:

• Let’s simulate Laue pattern for IrO₂ single crystal (right now I have the parameter for this compound, but you can try with any other compound). IrO₂ has tetragonal crystal structure, note the respective parameters in the following window:

• Here a, b, c are lattice constants. α, β, γ are angles between principle axes. Group is space group of the crystal structure. Dist.(Cm) is the distance between single crystal surface and Laue recording x-ray film or image plate. Angle(°) depends on the geometry of the Laue setup. In case of metals (like in our example IrO₂), we record the back-scattered x-ray pattern. In this case Angle 180°. In some cases of transparent single crystals, we record the transmitted pattern, in that case, Angle = 0°. λ1 and λ2 are lower and upper limit of x-ray source we are using for the experiment. Height and Width corresponds the height and width of the x-ray recording film or the dimensions of Image plate.

• You can save these data in a .dat file for future use. Just click the Save button on the top right of Data input window.

• Next time, if you like to load this saved data, choose File from Data menu, and navigate to the data file.

• Now we are ready to simulate the Laue pattern. First let’s see the Laue pattern for [100] crystallographic direction.

• Go to Matrix » Acquisition in the menu bar. You will see following window:

• Here we need to insert the crystallographic directions of our crystal we want to simulate. Here is an example:

• Please select Récipr. (stands for reciprocal space). Click OK, and here it is.

• Congratulations! Now you know how to simulate Laue pattern of any single crystal with known lattice parameters. Very simple, isn’t it? Now I will describe couple of useful techniques which you may find useful.

• Rotating the pattern to see the changes. Go to Simulation » Laue in the menu bar.

• Now you can put degree of rotation with respect to rotation axes and click Draw. That’s it.

• Another feature maybe be very useful: superimposing a simulated patter over the experimental pattern.

• Please save your experimental Laue pattern in .jpg (or another supported format). Note: sometimes I noticed that high resolution images show some scaling problem, in that case please scale down the resolution. The image file I am using is 625 pixels by 500 pixels.

• Go to menu bar: Coordinates » Image and browse your image file.

• Next we need to set the scale. Go to Scale » Measure, then I click at two points on the horizontal axis and enter the horizontal dimension of my image plate.

• Then we need to set the Origin, Go to Origin » Point, and click in the center of the film/pattern.

• In the main window, you should see something like this:

• That’s all from me. Now you may want to explore further yourself. Happy learning!

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