About cesium trichloromercurate(II) CsHgCl3: Solution of a complex superstructure and behaviour under high pressure

By solving the crystal structure of C5HgCl(3) a new uncommon distortion var iant of the cubic perovskite type with extremely (2 + 2 + 2)-distorted HgCl , octahedra has been found. The trigonal superstructure with space group P3 (2) and ninefold cell contents differs from the aristotype only so far, as 2/3 of the Cl-atoms are moved away from their ideal positions leading to 3 pairs of different Hg-Cl distances with about 2.35 Angstrom, 2.71 Angstrom and 3.15 Angstrom. The cations Cs+ and Hg2+ and the chloride ions with medi um Hg-Cl distance keep the ideal positions of a cubic perovskite lattice. D ue to the evenly distribution of the three different bonds in the three dir ections of cubic space the cell shows an almost perfect cubic metric. Raman spectra and powder diffraction experiments up to pressures of 5 GPa demons trated that the ideal perovskite arrangement is stabilized with increasing pressure. The shift of the FT-Raman bands show in agreement with spectra si mulations that the Hg-Cl bonds are equalized, leading to a regular octahedr al co-ordination of the Hg atoms. The disappearance of the Raman spectrum a t P>3.4 GPa indicates that the high pressure phase forms an ideal cubic per ovskite (a = 5.204(1) Angstrom,Hg-Cl - 2.60 Angstrom).