Local Structure in Amorphous Phases of Piroxicam from Powder X-ray Diffractometry

The local structures of amorphous phases of piroxicam were studied to explain differences in their recrystallization behavior, which we reported earlier (Sheth, A. R.; Bates, S.; Muller, F. X.; Grant, D. J. W. Polymorphism in piroxicam. Cryst. Growth Des., in press). The differences between the structures of form I ( P I ) at 25 and at 160 °C are consistent with anisotropic thermal expansion ( α x = 2.85 × 10 -5 K -1, α y = 1.96 × 10 -5 K -1, α z = 5.26 × 10 -6 K -1, volume thermal expansivity of the unit cell = 4.92 × 10 -5 K -1, increase in unit cell volume = 2.01%). Cryogenic grinding was employed to produce the respective amorphous forms, P AI from P I and P AII from P II . Pairwise distribution function (PDF) transforms were utilized to compare atom to atom correlations in P AI and P AII and to understand differences in their recrystallization behavior on the basis of their local structure. PDF transforms showed that P AI and P AII are types of amorphous material, which may be termed random ordered network (RON) amorphous. P AI and P AII were found to possess similar short-range interactions. The presence of P I -like residual long-range order in P AI explains recrystallization to P I . However, the absence of P II -like residual long-range order in amorphous P AII appears counterintuitive and suggests that P AII could recrystallize to a crystalline form different from the initial crystalline form.