AN X-RAY-DIFFRACTION STUDY ON THE TEMPERATURE AND CHEMICAL-COMPOSITION DEPENDENCE OF THE MODULATION WAVE-VECTOR IN [(CH3)(4)N]2ZNCL4-XBRX COMPOUNDS

In this paper we study the behaviour of the modulation wave vector in [(CH3)(4)N]2ZnCl4-xBrx compounds as a function of composition (x) and temperature. We compare the results of this x-ray study with those of morphological experiments. The two results are quite well correlated, showing several lock-in phases of the modulated structure being stable over a temperature range of a few degrees. In particular, the existen ce of a recently found new phase with modulation wave vector q = 4/11c is confirmed. We also find evidence for phases with q = 10/27c*, q = 14/37 (or 17/45)c, q = 14/39c*, q = 3/8c*, q = 5/12c*, q = 13/36 (or 17/47)c and q = 5/14 (or 16/45)c*. These phases are interpreted withi n the framework of a microscopic model.