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Glassy crystals III : X-ray determination of the structures of the plastic and glassy crystalline phases of cyclohexanol

1984; EDP Sciences; Volume: 45; Issue: 4 Linguagem: Francês

10.1051/jphys

2777-3396

Daniel André, D. Ceccaldi, H. Szwarc,

Liquid Crystal Research Advancements

Single-crystal X-ray diffraction data were collected and interpreted for plastic (I) and glassy crystalline (I' G ) phases of cyclohexanol. For phase I, the unit cell is face-centred cubic, the space group is Fm3m, (Z=4), with parameter a=8.809 A at 275 K. A Monte Carlo method coupled with a rigid group least squares refinement led to a structural model (weighted residual WR=1.05%) in which both cyclohexanol conformers have equal occupancy factors: each isomer can occupy 48 equiprobable orientations at each crystalline site. The mass centres are not fixed, the molecular symmetry plane does not coincide with a symmetry element of the space group and each conformer is characterized by the oxygen atom position in the crystalline cell. The quality of phase I' G single crystals is not as good as for phase I. But the same structural model fits the X-ray data (WR=2.6%) with a=8.608 A at 121 K. Comparison of these results with NMR data suggests that the same large amplitude motions exist in phases I and I' G , with much lower frequencies in the glassy crystal L'etude par diffraction RX montre que la maille cristalline de la phase plastique est cubique a faces centrees, de groupe Fm3m avec a=8,809 A. Description d'un modele structural ou les deux conformeres du cyclohexanol ont le meme taux d'occupation, chaque isomere pouvant occuper 48 orientations equiprobables en chaque site cristallin. Le meme modele semble convenir a la phase cristalline vitreuse. La comparaison des resultats a ceux obtenus par RMN suggere que les memes mouvements de grande amplitude existent dans les deux phases, meme si leurs frequences sont bien inferieures dans le cristal vitreux