Temperature dependence of the static distortion in incommensurate displacive phases and its effect on the diffraction pattern. I. Main reflections

The effect of the temperature dependence of the structural modulation on the intensity of main reflections is analyzed. A simple expression for the atomic scattering modulation factors has been derived describing the structural distortion in terms of the amplitude and the inhomogeneous phase of the order parameter. According to this expression, some main reflections can be strongly influenced by modulation harmonics. Owing to this influence, the intensity of some main reflections can increase, decrease or even show a pronounced minimum as the temperature is lowered. These results are experimentally verified in thiourea.