Structure of the modulation in thiourea. I. symmetry properties

2014 This is the first part of a structural study of the modulation in thiourea. The modulation is described by a normal mode Qq of 03C44 symmetry along with harmonic modes Q2q, Q3q, ... Corresponding displacements are derived in view of structure refinement. In commensurate phases, these give access to the space group; in incommensurate phases, to the supersymmetry. The superspace approach to the symmetry of the modulation is compared to the Landau theory approach. Discussion of the free energy shows that a ferroelastic lock-in phase may be stable under stress and gives information as to the phases of the component modes Qq, Q2q, ..., important for structure refinement. J. Physique 46 (1985) 2187-2195 DTCEMBRE 1985,