PURPOSE:
To expose the student to new equipment and lab procedures. And to determine the interplanar spacing of a sample using an x-ray diffractometer.

INTRODUCTION:
X-rays are electromagnetic radiation of wavelength about 1 Å (10^-10 m), which is about the same size as an atom. They occur in that portion of the electromagnetic spectrum between gamma-rays and the ultraviolet. The discovery of X-rays in 1895 enabled scientists to probe crystalline structure at the atomic level. X-ray diffraction has been in use in two main areas, for the fingerprint characterization of crystalline materials and the determination of their structure. Each crystalline solid has its unique characteristic X-ray powder pattern which may be used as a "fingerprint" for its identification. Once the material has been identified, X-ray crystallography may be used to determine its structure, i.e. how the atoms pack together in the crystalline state and what the interatomic distance and angle are etc. X-ray diffraction is one of the most important characterization tools used in solid state chemistry and materials science.

An x-ray diffractometer can be used to bombard a sample with x-rays and then detect the angle at which the x-rays bounce off (diffract). The x-rays are generated, filtered, strike the sample and are detected by the x-ray diffractometer. Below is a simplified x-ray diffractometer showing the basic parts.

The signal detected by the diffractometer is sent to a computer for analysis and then displayed on a cathode ray screen or printed graph.

The peaks on the graph can then be used to determine the interplanar spacing of a sample using Bragg's equation:

d = n x wavelength / 2 x sin(theta)

where d is the interplanar spacing, n is the order (assumed to be 1), wavelength is the wavelength of the x-rays used (commonly Cu at 1.542 Å) and theta is the angle at which the x-rays diffracted. The graph normally lists the angles of the peaks as degrees 2 theta, thus the angle theta is one half that which is shown on the graph.

During this lab you will operate a x-ray powder diffractometer to determine the first three interplanar spacing of a sample.

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PROCEDURE:

1) You can adjust the background shading by clicking on the "Special" button to the right and selecting "Background". Click on the "Special" button and select "Print Blank Report" to obtain a web page that can be printed and used as a lab report. (the program will not be interrupted.

2) Record the wavelength of the x-ray tube used in this x-ray diffractometer, it can be found listed on the x-ray tube.

3) Open the door on the sample chamber by clicking on the appropriate button. The sample has been prepared and placed on top of the unit on a small holder. Pick up the sample and place it in the now open sample chamber. Close the chamber door by clicking on it.

4) Turn on the power to the x-ray tube by clicking on the switch on the side of the power supply. Set the number of counts per second, on the graphing device, to about 100. By adjusting this lever, the size and location of the diffraction peaks can be changed to best display the data. A graph where the peaks just about touch the top of the paper is best.

5) Click on the scan button to start the sample analysis. While the scan is in progress, none of the adjustment buttons and levers will operate.

6) The graph should show three large peaks and several smaller ones. Record the degree 2 theta angles of the three largest peaks. On the graphing device there is a movable clear plastic strip with a red line index. Drag this strip over to each large peak to assist you in reading the peak's value.

7) Calculate the requested values asked for on the lab sheet and any given by your teacher. For help on these values click on the "Special" button and select "View Data & Hints". Select "File Report" to send a copy to be viewed by your teacher.