Halogen NQR and the Phase Transition in CH3Hg-Halide Family

CH3HgCl (methylmercury(II) chloride, MMC) is a toxic compound, representing also one of the initial steps in the metabolic chain for Hg poisoning. It belongs to the methylmercury(II) halide family with relatively uncomplicated molecular composition which has a simple scheme of molecular vibrations with a single torsional mode. These compounds are known to exhibit different structural phase transitions [1,2] in the solid state and are a good example for studying and modelling this type of phase transitions. The aim of this research is to use CI NQR to examine the hypothesis of analogous sequence of isostructural phases at ambient pressure in the methylmercury(II) chloride, bromide and iodide [2]. This hypothesis has been made mainly on the basis of Raman measurements, available structural data and space group analysis. The room temperature structure of MMC shown on Fig. 1 is tetragonal, space group P4/nmm [2, 3]. It is supposed that below 162 K a displacement phase transition, driven by the softening of a phonon at the Brillouin zone boundary, occurs. The linear molecules CH3HgCl move out of the mirror planes and the unit cell doubles. The low temperature structure of MMC is presumably isostructural to the room temperature y A x