Spectroscopy at very high pressures. Part II. Far-infrared spectra of some complex chlorides MI2MCl6

Abstract

Far-i.r. absorption spectra of some hexachlorometallates, M^I₂MCl₆, have been studied at average pressures up to 20 kbar by use of a new design of diamond anvil cell. No simple pattern of behaviour emerged although the ν₄, δ(ClMCl), mode is generally much more pressure-sensitive than the corresponding ν₃, ν(M–Cl). Lattice modes usually have dν/dP≈ 1·0 kbar cm^–1, but exceptions were found. For (NH₄)₂[PtCl₆] and (NH₄)₂[SnCl₆]ν₄ and ν_L are in Fermi resonance which can be pressure-scanned to reveal different apparent sensitivities of ν₄ and ν_L; the results are analysed to obtain true shifts. Finally, in hexachlorotellurates the vanishingly weak ν₄ gained intensity dramatically as pressure was applied; a possible mechanism is discussed.