Preparation of F-centers by Powder X-ray Diffraction
This experiment was developed by Paul Hansen and George Lisensky.
Shining x-rays on alkali halides excites electrons
into halide vacancies (an F-center). The observed color depends on the size
of the vacancy which depends on the size of the unit cell. Read more information about F-centers.
Prepare a microscope slide with an aluminum holder. Take a piece of tape
and wrap it around the slide so that the sticky side is facing out. Place
the aluminum holder on top of the slide with the tape centered in the
hole. The end of the aluminum must match the end of the slide.
Since the instrument is a powder x-ray diffractometer your sample should
be a powder. Grind the sample using a rotary motion with a mortar
and pestle. The finer the powder the better the data.
Pour the powder on to the holder. Use another microscope slide to wipe
off the excess. Best results are obtained if the sample is level and as
deep as the holder.
When you turn the latch to open the door the yellow light labeled “shutter
open” will turn off. This is because turning the latch closes
the shutter between the x-ray tube and the sample. With the shutter
closed x-rays will not be present.
Make sure the plastic tray is under the sample. This
container is meant to catch any powder which could otherwise fall to
the x-ray source and damage the machine. Hold both the slide and the
aluminum holder. Insert the pair together into the clip holder.
Run a powder diffraction scan of an alkali halide. (The sample must
be tilted to get a direct beam on the sample and create F-centers.) At
the end of the scan, open the door to release the shutter and block the
x-rays. Immediately observe the color of the irradiated area.
Different alkali halides have different color F-centers. (F is an abbreviation
for Farbe, the German word for color.)
How long does it take, as illustrated by the loss of color, for the
electron in the F-center to return to its origin? Do different materials
take the same time?
1. Predict the relative sizes of the KBr, KCl, and NaCl unit cells.
2. The tallest peak is the 200 reflection. What is your measured size
of the unit cell?
3. What are the F-center colors for each of these?
4. Does the spectral order of the colors match the unit cell sizes?