Clear Science!

Every material that has atoms with order or crystallinity has a characteristic plot of intensity versus angle 2θ when you shine an X-ray beam through it. This is because of how the atoms stack on each other in the material. And so: if you have a material you can’t identify, you can shine an X-ray beam through it and compare the plot to standard plots to figure out what material it is.

This is called X-ray diffraction or XRD. Up above you see the XRD patterns for gold, silver, and one kind of iron oxide. You can also do a little math and figure out the actual arrangement of the atoms from these plots. This is how we know the structure of many things, for example rubies.

A beam of X-ray light is going through our battery, and an X-ray detector is on the other side. We’ve drawn lines to show how the angle between the beam and the detector can change by moving the detector. We call that angle 2θ or “two-theta.”

If the detector gets to a value of 2θ where X-rays get reflected, it gets a flash of intensity. The plot shows the intensity at the detector (y-axis) vs. the angle 2θ (x-axis). Every time there’s a spike or peak, that’s where more X-rays get reflected and the detector sees a flash there.

We talked about how light reflects off things, and how if you looked at a mirror at different angles you might get a blast of reflected light in your eyes. We also talked about how we were using light reflections to study batteries. How do they tie together?

Instead of your eyes use a detector, which is just something that senses light (like your eyes do). A beam of light (X-rays) goes through a battery with a hole in it. Most of the X-ray light goes straight through, but some of it hits the atoms in the battery material and bounces off. So the atoms are like little mirrors. And the detector finds out what angles the light gets bounced off at.

This is an “in situ” experiment because you can discharge and charge the battery while the experiment is happening. In situ is a fancy word scientists like to say that is Latin for “in position.” What it really means is that you don’t have to discharge the battery then take it somewhere else to shine light through it. It all happens in one place. (The opposite of in situ is “ex situ.”)

A synchrotron light source is a giant scientific facility made to generate X-rays. We wondered what X-rays could be useful for. It turns out they have just the right wavelength to figure out the location of atoms in a solid material.

X-rays with their wavelengths lined up strike the sample material at some angle, and they get bounced off elastically by the electrons in the atoms. Then you detect the bounced off X-rays at the same angle.

At some special angles, X-rays bouncing off different atoms will overlap, but their wavelengths might not line up anymore. You Clear Scientists know that overlapping light waves interfere with each other. And from this interference, you can use geometry to figure out the atomic spacing (5 nm in this example).