Study of the low-temperature phase transition in Hg2Cl2

Studies of the temperature dependence of the X-ray powder diffraction and single-crystal Raman spectra of Hg2Cl2 are reported for temperatures from 300K to below the phase transition at approximately 180K. From the X-ray data it is established that the crystal class changes from tetragonal in the high-temperature phase to monoclinic in the low-temperature phase. The Raman spectra at low temperatures are explained by a primitive monoclinic cell of doubled content and the space group identified as C2h2. The temperature dependence of the Raman spectrum of a 'soft' acoustic branch is observed above the phase transition and used to suggest a mechanism for the transition.