Abstract
A reinvestigation of the modulation parameter δ in Rb2ZnCl4 with the double axis X-ray diffractometer “AMADEUS” revealed a discontinuous transition from the incommensurate phase to the ferroelectric one. In a region of about 1.5 degrees, the incommensurate and ferroelectric Bragg-reflections coexist. The half-width of the satellite reflection increases considerably in the vicinity of Tc whereas that of the commensurate one remains constant.