title: Ab initio structure determination of the high-temperature phase of anhydrous caffeine by X-ray powder diffraction.
abstract: The high-temperature phase I of anhydrous caffeine was obtained by heating and annealing the purified commercial form II at 450 K. This phase I can be maintained at low temperature in a metastable state. A powder X-ray diffraction pattern was recorded at 278 K with a laboratory diffractometer equipped with an INEL curved position-sensitive detector CPS120. Phase I is dynamically orientationally disordered (the so-called plastic phase). The Rietveld refinements were achieved with rigid-body constraints. It was assumed that on each site, a molecule can adopt three preferential orientations with equal occupation probability. Under a deep undercooling of phase I, below 250 K, the metastable state enters in a glassy crystal state.
authors.affiliation: Laboratoire de Dynamique et Structure des Matériaux Moléculaires (UMR CNRS 8024), UFR de Physique, Bâtiment P5, Université des Sciences et Technologies de Lille, 59655 Villeneuve d'Ascq CEDEX, France. patrick.derollez@univ-lille1.fr
authors.forename: Patrick
authors.initials: P
authors.lastname: Derollez
authors.forename: Natália T
authors.initials: NT
authors.lastname: Correia
authors.forename: Florence
authors.initials: F
authors.lastname: Danède
authors.forename: Frédéric
authors.initials: F
authors.lastname: Capet
authors.forename: Frédéric
authors.initials: F
authors.lastname: Affouard
authors.forename: Jacques
authors.initials: J
authors.lastname: Lefebvre
authors.forename: Marc
authors.initials: M
authors.lastname: Descamps
mesh_terms: D002110:Caffeine / Q000737:chemistry*; D018360:Crystallography, X-Ray; D008958:Models, Molecular; D015394:Molecular Structure; D033362:Powder Diffraction; D013696:Temperature; D014961:X-Ray Diffraction
chemical_list: D002110:Caffeine