Structural analysis of Cs2HgBr4 in normal, incommensurate and twinned phases

X-ray diffraction and Raman spectroscopy experiments have been used to investigate different phases of Cs2HgBr4, dicesium mercury tetrabromide, from room temperature to 213 K. Structural analyses have shown that the crystal could be described, in the normal and in the incommensurate phases, both by ordered and disordered models, but the latter gave more consistent results. Raman results corroborate the descriptions based on X-ray analysis; the presence of an extra peak, which according to group theory should be forbidden in an ordered structure, indicates the lack of local symmetry and was associated with an orientational disorder of [HgBr4](2-) tetrahedra. In the transition from the incommensurate to the commensurate phase a multi-soliton behavior was observed. The Cs2HgBr4 crystal in the low-temperature commensurate phase is composed of two types of ordered pseudomerohedral twinned domains.