BIREFRINGENCE, X-RAY-DIFFRACTION AND RAMAN STUDIES IN THE ONE-DIMENSIONAL COMPOUND CH3NH3HGBRCL2

Monomethyl ammonium monobromo-dichloromercurate exhibits a structural phase transition at 291 K: Phase I - (291+/-5K) --> Phase II This tran sition has been characterized by optical birefringence, Raman scatteri ng on polycrystalline samples and X-ray measurements. The space groups and the cell parameters of both phases were determined by X-ray diffr action from single crystals and powder samples. Phase I has space grou p P3(2) (a = b = 7.989(2) Angstrom; c = 9.646(2) Angstrom; Z = 3). Pha se II presents a high degree of disorder with monoclinic symmetry (spa ce group Cm with a = 13.846(6); b = 7.898(2); c = 9.748(6) Angstrom; b eta = 90.25(1); Z = 6).