THE TEMPERATURE AND CHEMICAL-COMPOSITION DEPENDENCE OF THE MODULATION WAVE VECTOR IN ((CH3)4N)2ZNCL4-XBRX COMPOUNDS DETERMINED FROM THE MORPHOLOGY

In this paper we investigate the morphology of ((CH3)4N)2ZnCl4-xBrx (x = 0 to 4) solid solution crystals in the incommensurate and lock-in phases by sphere growth experiments. On these spheres satellite faces appear, which give direct information about the crystal structure. The crystal structure can be described by a normal basic structure distorted by a displacive modulation with wave vector q, added to the classical 3D lattice. The sphere experiments reveal directly the phase transitions in the crystal structure as well as the magnitude of the modulation wave vector. As a result, we confirm the data on q as determined in X-ray diffraction experiments, and find in addition a new commensurate phase (q = 4/11 c*) for x = 1.2 to 2.19 in between two incommensurate phases. We discuss the possible symmetry of this phase.