The origin of disorder in CH3HgX (X = Cl, Br and I) crystals investigated by temperature dependent Raman spectroscopy

Methyl-mercury(II) halides CH3HgX (X = Cl, Br and I) were studied by means of temperature dependent Raman spectroscopy from 10 K to 410 K. In addition to the previously reported soft phonons, new changes in the low frequency spectra were observed at T approximate to 70 K in CH3HgBr and at Tapproximate to 100 K in CH3HgI. The bandwidths of the two internal modes in CH3HgBr, the CH3 symmetric stretching band and the C-Hg-Br bending band, rise towards a local maximum at T approximate to 50 K as the temperature is raised from 10 K to 300 K. On the other hand the bandwidths of the two corresponding modes in CH3Hgl crystals monotonously increase with temperature, obeying an Arrhenius law. Besides the three phonon modes present in theRaman spectra of CH3HgCl at room temperature, the fourth phonon band that has been observed at temperatures below 245 K might correspond to the freezing of methyl librations. The huge bandwidth of the C-Hg-Br bending mode could suggest the presence of additional weak bonding of a mercury atom with bromine atoms from other molecules, thus inducing positional disorder