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

Abstract

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 (PI) at 25 and at 160 °C are consistent with anisotropic thermal expansion (αx = 2.85 × 10-5 K-1, αy = 1.96 × 10-5 K-1, αz = 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, PAI from PI and PAII from PII. Pairwise distribution function (PDF) transforms were utilized to compare atom to atom correlations in PAI and PAII and to understand differences in their recrystallization behavior on the basis of their local structure. PDF transforms showed that PAI and PAII are types of amorphous material, which may be termed random ordered network (RON) amorphous. ...