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

Abstract

Abstract 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 P1̅(αβγ) with the modulation vector q⃗ = 1/3·a⃗*. 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 P1̅. 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.