I'm confused about the relationship between the theta and 2theta values when using an XRD to scan materials. I did a scan on a wafer (not powder XRD) and found a 2theta peak at approximately 66 degrees, with theta at 33 degrees. This makes sense - from my understanding, 2theta should always be double theta.
However, when I did a scan on a similar material, I got strange results. When I held 2theta at 66 degrees and did a 2theta/omega scan with 2theta at 66 degrees and theta at 33 degrees, I saw no peaks. I found a rocking curve peak at theta = 27 degrees, so I tried a 2theta/omega with 2theta/theta at 54 and 27 degrees respectively - that showed no peak.
Finally, when I did a 2theta/omega scan with 2theta at 66 degrees and theta at 27 degrees, I saw a clear peak.
Am I misunderstanding something about how an XRD functions? What is going on with this?
In a Bragg-Brentano diffractometer, the detector arm should always be at the same angle to the surface of the specimen as the source arm. This gives the conventional Th/2Th arrangement. The side-effect of this is that only planes parallel to the specimen surface will diffract. I'm assuming the wafer was a single crystal? In that case, you will only get diffraction from the wafer if the plane it was cut along has a non-zero structure factor. If the "similar material" was another wafer, then if it was cut along a different direction, then the peaks will be in a different spot because different planes are parallel to the surface.
I think you are having issues because you're using a single-crystal and not a powder.
In a Bragg-Brentano diffractometer, the detector arm should always be at the same angle to the surface of the specimen as the source arm. This gives the conventional Th/2Th arrangement. The side-effect of this is that only planes parallel to the specimen surface will diffract. I'm assuming the wafer was a single crystal? In that case, you will only get diffraction from the wafer if the plane it was cut along has a non-zero structure factor. If the "similar material" was another wafer, then if it was cut along a different direction, then the peaks will be in a different spot because different planes are parallel to the surface.
I think you are having issues because you're using a single-crystal and not a powder.
Matthew R. Rowles has pretty much nailed it. Your wafer is not cut with the plane corresponding to the Bragg peak at 66deg 2theta oriented normal to the surface. So for conventional 2th:th scans, this peak is not seen. But if you tilt the crystal by adjusting omega you can potentially bring these planes into the Bragg condition.
A powder diffractometer is not really designed to study single crystals (or single crystal wafers). But we take advantage of this effect when making "zero background plates", which are cut off axis exactly to avoid getting reflections for the wafer.
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