Structure solution and refinement from powder or single-crystal diffraction data? Pros and cons: An example of the high-pressureβ′-polymorph of glycine

Abstract

The structure of a high-pressure polymorph of glycine (theβ′-polymorph formed reversibly at 0.8 GPa from theβ-polymorph) was determined from high-resolution X-ray powder diffraction data collectedin situin a diamond anvil cell at nine pressure points up to 2.6 GPa. X-ray powder diffraction study gave a structural model of at least the same quality as that obtained from a single-crystal diffraction experiment. The difference between the powder-diffraction and the single-crystal models is related to the orientation of the NH3-tails and the structure of the hydrogen-bonds network. The phase transition between theβ- andβ′-polymorphs is reversible and preserves a single crystal intact. No transformations were observed between theβ-,α-, andβ′-polymorphs on compression and decompression, although theα- andβ′-polymorphs belong to the same space group (P21/c). The instability of theβ- andγ-forms with pressure can be predicted easily when considering the densities of their structures versus pressure. The direction of the transformation (i.e., which of the high-pressure polymorphs is formed) is determined by structural filiation between the parent and the high-pressure phases because of the kinetic control of the transformations.