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

The room temperature structure (a-form) of the organic compound p-chlorobenzamide, C7H6ClNO, can be described as a classical three-dimensional periodic superstructure (alpha(s)) and also as a commensurately modulated structure (alpha(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) over bar(alphabetagamma) with the modulation vector (q) over right arrow = 1/3 . (d) over right arrow*. 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 (gamma-form) contains one molecule in the asymmetric unit, the space group is P (1) over bar. The high temperature structure (gamma) 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.