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A. Schönleber, Philip Pattison, Gervais Chapuis

The (3 + 1)-dimensional superspace description of the commensurately modulated structure of p-chlorobenzamide (α-form) and its relation to the γ-form

The room temperature structure (α-form) of the organic compound p-chlorobenzamide, C₇H₆ClNO, can be described as a classical three-dimensional periodic superstructure (α_S) and also as a commensurately modulated structure (α_M) in (3 + 1)-dimensional superspace. The diffraction pattern is characterized by a clear difference in intensity between main and satellite reflections. All reflections can be indexed with four indices hklm in a triclinic unit cell and one modulation vector in the superspace group P-1(αβγ) with the modulation vector q = 1/3 · a*. The structure undergoes a phase transition at higher temperature. In this phase transition the modulation vanishes as indicated by the disappearance of the satellite reflections. The resulting high temperature phase (γ-form) contains one molecule in the asymmetric unit, the space group is P-1. The high temperature structure (γ) is very close to the reference structure of the modulated model at room temperature. The phase transition can therefore be understood as a loss of the modulation at high temperature.

Zeitschrift für Kristallographie, Oldenbourg Wissenschaftsverlag

Print ISSN: 0044-2968
Volume: 218, 07/2003
Pages: 507

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