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I'm looking at a $\theta$ - 2$\theta$ pattern of my thin film which in bulk is cubic (bcc) and I see 001 and 002 peaks of the film. There is supposed to be a tetragonal distortion meaning that I need to find the lattice parameters a and c. Now from this data I can calculate c, but to find a I read that I need to measure an off-axis peak but I don't know what that means.

In practical terms I understand that I need to run another scan so that I find an h k l peak such as 101 or 202 to be able to solve for a, but in diffraction knowledge terms I don't know what an off-axi peak is.

I looked through my copy of Cullity and there is no mention once of an off-axis peak, so I don't know where else to look.

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In some sense, you have answered your own question. All this means is that you cannot determine $a$ from a series of reflections along the same axis (e.g. ($001$), ($002$), ($003$), etc.). These are all along that same direction in reciprocal space, so therefore they are all on the same axis. If you measure an off-axis reflection, you will able to calculate $a$ and $c$. The important point is that the axes referred to here are in reciprocal space.

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Measuring an off-axis peak simply means that you're looking for crystal planes, which are not parallel to the sample surface. Therefore, they have an in-plane component (in addition to the perpendicular lattice plane spacing, which is commonly measured), which is the part, you're looking for.

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