XRD is the process by which reflected X-Rays interfere with each other when reflected off the different layers within a crystal
The general rule is that \(n\lambda=2sin\theta\), which relates the distance between the two layers reflecting the xrays to the wavelength of the xrays bombarding the crystal.
XRD can be used to analyse both a single crystal and a crystalline powder.
Since the orientation of the crystals can’t be aligned in a powder, the resulting diffraction pattern becomes a series of concentric circles.
Single crystal spectra can be used to determine the absolute structure of a unit cell, and therefore the molecule
Powder diffraction is used more as a profiling/fingerprinting technique