Lecture #15, Friday, May 1st

Can you really believe we are half way through the quarter already?

Today we discussed more about X-ray diffraction and went over some key details of what you need in order to interpret an XRD pattern.

WHEN INTERPRETING AN XRD PATTERN YOU MUST ACCOUNT FOR EVERY SINGLE PEAK IN THE PATTERN!

I cannot stress that point enough. When we are analyzing whether a solid solution forms or whether the compound is a two phase mixture we need to look at all the peaks in the XRD pattern. Further analysis of the pattern tells us how the unit cell is changing when we replace one ion with another. This leads to an important relationship from Bragg's Law:

As the spacing between planes gets larger the peaks in an XRD pattern shift to lower angle, or shift to the left.

The same relationship holds when the spacing between planes gets smaller, as the peaks shift to the right.

As a chemist, we can manipulate the spacing between the planes by substituting a cation/anion with a smaller or larger atomic radii. By doing this we will be able to influence the bonding and more importantly the color of a compound.

Another thing about X-ray diffraction is that you need to have a crystalline sample in order to obtain a diffraction pattern. When I was talking about X-ray diffraction last quarter I received this e-mail from former student Asher Kay:

"I was watching Alton Brown's Good eats on the food network and he was making fudge. He wanted to study the crystal structure of the finished fudge and got a person who studies crystal structures( I can't remember the name) to use an X-ray Diffractometer, and Bragg's law to study the diffraction pattern. As it turns out the structure is identical to the sugar crystals that were used to make the fudge. Actually, making fudge is a very chemically intensive task. It is very important to control the temperature to make sure the super saturated solution of fudge does not re precipitate in the form of grainy crystals. Also, you cannot allow crystals to form on the sides of the pot or it will be all over."

When the X-ray patterns of the fudge were analyzed, it didn't matter if you had chocolate fudge, peanut butter fudge, or any combination of the two, all the XRD patterns looked the same. The reason for this is that the only crystalline material in the fudge is sugar. Therefore all the peaks will look the same even though the fudge was different. This story helps emphasize what information we can obtain from an X-ray diffractometer. We can tell which crystal structures are present and the only samples we can use are crystalline materials.

On Monday we will have a special guest in class and I will finish up on solid solutions and emphasize how they relate to a phase diagram.

Hope everyone has a great weekend, Dr. Fus