Integrating Research and Education > Crystallography > Tutorials > Using XtalDraw to Make Animated GIF

Using XtalDraw to Make Animated GIF's: The High-Temperature Phase Transition of Quartz

Robert Downs, University of Arizona

One of the most famous and most studied phase transitions in minerals is the transformation of quartz from the α to the β phase as a function of temperature. At around 843 K, the symmetry of quartz changes from trigonal to hexagonal in what is known as a displacive phase transition in which no bonds are broken. The high-temperature form is not quenchable, meaning that as temperature drops, β quartz always transforms to α quartz. Kihara, in 1990, conducted the best X-ray diffraction study of the evolution of quartz from room temperature through the α-β transformation to 1078 K. In this exercise, we will create a movie that shows the change of quartz as a function of temperature, through the α-β transition using the published data of Kihara (1990). The following instructions are for XtalDraw.
An interesting exercise to try is to make the same sequence of frames but instead of displaying the atoms in polygon mode, choose to view them as displacement ellipsoids. The ellipsoids enclose the region of space that has a certain probability of finding the atom as it vibrates to and fro with thermal motion. As a function of temperature the ellipsoids will increase in size and the mechanism for the phase transition should become clearer. A careful reading of Kihara's paper will provide an understanding of the crystallographic aspects of this interesting phase transition. Helen Megaw's book includes a well written discussion of this transformation. Downs (2000) has an in depth review of displacement ellipsoids that uses Kihara's quartz data as an example to gain a better understanding of the effect of temperature on the motion of atoms in silica. It also explains how thermal motion can affect bond lengths.

The animation viewed in XtalDraw is a simple process that shows bitmaps one after another in the window. It is not something that can be ported to a web browser or to Powerpoint for a class or presentation. So, in this final section of the discussion, a routine to produce these options will be described. The individual bitmaps shown in the XtalDraw animation are stored in the directory that you defined when you created the movie file. These bitmaps were generated with sequential names that can be easily ordered and input into a animated gif program. For instance, GIF Movie Gear is an inexpensive program that has a free trial version and is quite easy to use. You can find them at http://www.gamani.com/ to test their software. Simply open all the bitmaps simultaneously into their program and hit play, and in three keystrokes you have created an animated gif file that works both in Powerpoint and in a web browser.

References

Downs, R.T. (2000) Analysis of harmonic displacement factors. Reviews in Mineralogy and Geochemistry, 41, High-Temperature and High-Pressure Crystal Chemistry, Robert M. Hazen and Robert T. Downs, Editors. Mineralogical Society of America, Washington DC.

Kihara, K. (1990) An X-ray study of the temperature dependence of the quartz structure. European Journal of Mineralogy 2, 63-77.

Megaw, H.D. (1973) Crystal Structures: A working approach. W.B. Sanders Company. 563 pp.

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