HIGH-ORDER DIFFRACTION SATELLITES AND TEMPERATURE-VARIATION OF THE MODULATION IN THE INCOMMENSURATE PHASE OF RB2ZNCL4

The intensity of a set of main reflections and satellites up to sevent h order has been measured as a function of temperature within the inco mmensurate phase of Rb2ZnCl4. The observed temperature dependence of t he diffraction peaks can be explained by a variation of the static str uctural modulation through the changes in 3 parameters: the amplitude of the primary mode, the soliton density that describes its anharmonic ity, and the amplitude of a third harmonic modulation. The eigenvector s of the primary mode and the third harmonic used for the static modul ation have been derived from previously published structural data on t he incommensurate and the ferroelectric structure. The soliton density behaviour, determined for the first time from X-ray diffraction data, agrees with results derived from other techniques.