TIME RELAXATION OF THE LOW-TEMPERATURE MODULATED STRUCTURE OF THIOUREA

A conspicuous time variation of the diffraction diagram of thiourea in its incommensurate phase at 170 K is observed and characterized. The phenomenon is systematic and reproducible. Satellite intensities decre ase according to linear laws, reaching in the case of third-order sate llites up to a 28% reduction. Main reflections decrease or increase de pending on the particular reflection. Sample degradation could be disc arded as a possible cause. Through a rather complex procedure, which m akes use of the structural data determined in previous studies, the ki netic process observed is shown to be fully consistent with a slow tim e relaxation of the soliton density of the incommensurate structural m odulation from a value of 0.62 at the beginning of the measurement to 0.69 at the end of the main experiment, after 166 h. This variation re presents a correlated decrease in the amplitudes of the harmonics of t he modulation, with a reduction of its anharmonicity. Clearly, the str uctural modulation evolves with time towards a more sinusoidal regime. The origin of the phenomenon is unclear and its possible existence in other incommensurate materials is to be ascertained.