Temperature dependence of the static distortion in incommensurate displacive phases and its effect on the diffraction pattern .2. Satellite reflections

The effect on satellite reflections of the temperature variation of the static structural modulation in incommensurate phases is studied. A structural model is introduced that includes the anharmonicity described by the soliton density and follows the assumptions and predictions of the Landau theory generalized to incommensurate phases. Within these premises, a simulation of the diffraction pattern as a function of temperature of Rb2ZnCl4 has been carried out. It is shown that the intensity of satellites does not always decrease monotonically with their order and its temperature dependence varies, depending on the satellite, so that it cannot be reduced to a common effective power law. Satellites of first order, for instance, can decrease or increase near the lock-in transition, depending on the particular reflection considered. A simple method for measuring the soliton density is proposed that is based on the expected correlation of the intensities of close first-order and higher-order satellites. Finally, the effect that high-order harmonics can have in a standard refinement of an incommensurate structure near the lock-in transition is discussed.