ζ-Glycine: insight into the mechanism of a polymorphic phase transition

• Main Authors: Craig L. Bull, Giles Flowitt-Hill, Stefano de Gironcoli, Emine Küçükbenli, Simon Parsons, Cong Huy Pham, Helen Y. Playford, Matthew G. Tucker
• Format: Article
• Language: English
• Published: International Union of Crystallography 2017-09-01
• Series: IUCrJ
• Subjects: amino acids; crystal structure prediction; polymorphism; neutron diffraction; phase transitions; crystallization under non-ambient conditions
• Online Access: http://scripts.iucr.org/cgi-bin/paper?S205225251701096X

Glycine is the simplest and most polymorphic amino acid, with five phases having been structurally characterized at atmospheric or high pressure. A sixth form, the elusive ζ phase, was discovered over a decade ago as a short-lived intermediate which formed as the high-pressure ∊ phase transformed to the γ form on decompression. However, its structure has remained unsolved. We now report the structure of the ζ phase, which was trapped at 100 K enabling neutron powder diffraction data to be obtained. The structure was solved using the results of a crystal structure prediction procedure based on fully ab initio energy calculations combined with a genetic algorithm for searching phase space. We show that the fate of ζ-glycine depends on its thermal history: although at room temperature it transforms back to the γ phase, warming the sample from 100 K to room temperature yielded β-glycine, the least stable of the known ambient-pressure polymorphs.