Vibrational spectroscopy at very high pressures. XXXXVIII. A Raman study of the phase behaviour of CH3Hg (X=Cl,Br,I), and the crystal structure of CH3HgBr

Raman spectra of CH3Hg (X=Cl,Br,I) have been studied as a function of pressure using diamond anvil cells. Phase transitions were discovered in the chloride at 1.5+or-0.3 and 5.9 kbar, with an almost immediate further transition near 6 kbar to a phase stable to at least 40 kbar. CH3HgBr shows a single transition at 12.5 kbar. Soft modes were found for all three materials. That for the bromide was followed by change of temperature as well as of pressure. The crystal structure of CH3HgBr was found to be like that of the chloride, but of a different space group (P421m, Z=4), allowing slight nonlinearity of the C-Hg-Br skeleton. On the basis of this structural evidence a phase diagram is proposed. A very narrow (about 0.1 kbar) phase in CH3HgCl may be incommensurate. Preliminary studies of CH3HgI show that its behaviour parallels that of its congeners, but that the crystallography is different in detail.