Single crystal pattern is a picture of point lattices, while polycrystalline is picture with several circles sharing the same center.

First record the XRD pattern of a single crystal on a CCD or imaging plate with a polychromatic X-ray and you will get dots in both transmitted and backscattered conditions. This is called as Laue's diffraction. This is basically, explained by Ewalds' sphere condition. That is the reciprocal lattice points that are falling on the Ewald's sphere with a radius 1/lambda will produce each diffraction point.

Now, slightly powder the same sample and record the XRD pattern with monochromatic X-rays as mentioned above in the transmitted geometry, we get concentric circles as dots. Now, finely powder the crystal and record the XRD using a monochromatic X-ray, which will produce complete circles of concentric, i.e. all the points connected and form the completed circle. That is the large ensembles or crystallites are available uniformly in all the circles which makes the completed circle. This can be explained based on the principle of Bragg's law. In the absence of well-powdered crystals with monochromatic X-rays, only a few planes are satisfying Bragg's condition form curves. For practical purposes for identifying the phase purity and crystal structure powder diffraction method is very handy.

I'll try to be as brief as possible.

Single Xtal materials reveal spot diffraction patterns while polyXtals exhibit ing diffraction patterns. Ring DP's varies like spotty rings, continious rings or dashed rings depending on the crystal sizes.

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The main difference between the single-crystal XRD and the Powder XRD is; that one can find the molecular structure and crystal structure including organic crystals in the single-crystal XRD. Particularly the 'H' atom position can also be identified accurately to some extent. Whereas in the powder XRD one can find the crystal structure, not the molecular structure. Single crystal diffractometer is based on muti-circular goniometers like 3-axis, 4-axis, 6-axis, etc. In a single crystal XRD, one has to find the various angles like phi, chi, omega, and two-theta for a particular plane, and then for the other planes also one has to calculate these values and acquire the data which is finally analyzed to find the crystal structure and molecular structure. Powder XRD-based only only one circle in the same circle both the source and detector will be there.