Why are we taking 2$\theta$ instead of $\theta$ in X-ray powder diffraction (XRD). I have found the forum post 2 theta in X-ray Powder Diffraction (XRD), but there is no answer. What is the explanation?
For the $\theta : 2\theta$ goniometer, the X-ray tube is stationary, the sample moves by the angle $\theta$ and the detector simultaneously moves by the angle $2\theta$. At high values of $\theta$, small or loosely packed samples may have a tendency to fall off the sample holder.
When you do diffraction, $\theta$ is the angle of incoming EM wave, as well as the angle of difracted EM in regard to Bragg's planes. So the total change in angle of the EM wave equals $2\theta$.
See images at http://en.wikipedia.org/wiki/Bragg%27s_law.
If this is not the answer you're looking for, maybe you should specify your question more clearly. (I was doing Bragg's experiments as a student decades ago, I naturally always halfed the angle, so I do not quite understand what is your problem.)
In x-ray powder diffraction, you have crystallites in all possible orientations. Only those crystallites whose bragg planes are at an angle θ with respect to the incident angle will diffract at an angle 2θ with respect to the incident beam (or at an angle θ with respect to the diffracting planes). So that is the reason, you always use 2θ instead of θ.