Identification of Crystalline Unit Cell
Indexing and unit cell determination for patent applications and structural characterization
Overview
In many cases it is important to identify the lattice type and dimensions of a crystallographic unit cell for an unknown material. The method for this, which is based on the determination of the crystallographic cell from an XRPD pattern, is called "indexing".
If all observed peak positions match the peak positions predicted from the crystallographic cell, this serves as a solid proof confirming the purity of a substance and ensuring that it is not a mixture of polymorphic forms. When all the peaks calculated from the cell parameters of a single crystalline form have been explained, a drug substance can be uniquely fingerprinted and considered to be a pure crystalline form.
This form of crystallographic analysis is particularly useful for the characterization of polymorphs. An indexed XRPD pattern or crystallographic cell parameters are often of critical importance in securing a patent.