INFLUENCE OF INCOMMENSURATE MODULATION ON THE OPTICAL-PROPERTIES OF THE SOLID-SOLUTION ((CH3)(4)N)(2)ZNCL2.8BR1.2

This paper reports on measurements with a high-accuracy universal pola rimeter of the birefringence, optical activity, and indicatrix relatio n in incommensurately modulated ((CH3)(4)N)(2)ZnCl2.8Br1.2 crystals. I t is shown that in this solid solution the inversion symmetry is broke n already in the paraelastic phase, due to the inhomogeneous distribut ion of CT and Br over the zinc-halide tetrahedra. This, however, allow s one to study the influence of the incommensurate modulation on a non zero optical activity. This influence is clearly shown for a sample in which, moreover, the modulation wave vector is believed to lock at se veral commensurate values.