The polycrystalline phenomenon of drugs refers to the formation of different crystal states of a compound molecule in the solid state due to the different crystal arrangement and filling methods. At present, the more mature methods for the identification of drug crystal types are: XRD, DSC, TGA, etc.

X-ray diffraction is the main means to study the crystalline form of drugs. X-ray diffraction can be used to distinguish the crystalline form from the amorphous state, to identify the varieties of crystals, to distinguish mixtures and compounds, to determine the crystalline structure of drugs, to determine the cell parameters, and to compare different crystalline forms.

Powder diffraction is the most commonly used method to study the polycrystalline form of drugs. The object of powder diffraction is not a single crystal, but the sum of many small crystals with random orientations. The powder X-ray diffraction pattern of each kind of crystal is like a person's fingerprint, and the intensity and distribution of diffraction lines of each kind of crystal measured by this method have special rules. From the patterns measured by powder diffraction, information such as crystal type change, crystallinity, crystal state and whether there are mixed crystals can be obtained.