A new X-ray determination of the modulated structure of thiourea

Abstract

Despite many previous investigations, the symmetry and structure of thiourea in its modulated configuration is still under discussion. We report here a new determination of this structure by means of X-ray diffraction, using a rigid-body model for the molecular units. The diffraction diagram up to third order satellites was measured in the commensurate phase with q=1/9b*, the structure refinement being performed however in the superspace formalism. The superspace group P(Pnma):(sT1) describes adequately the structure symmetry, allowing excellent results in the refinement. The anharmonicity of the modulation functions is considerable, with non negligible contributions of second and third harmonics. Despite theoretical predictions on the contrary, the form of the rigid-body modulation functions indicate a certain soliton regime. At a final stage of the refinement the possible modulation of the atomic thermal tensors was also considered. The results evidence a significant second harmonic modulation of thermal parameters.