Spectroscopy at very high pressures. Part 11. A Raman study of polymorphs of carbon tetrachloride and tetrabromide and their photodecomposition products

Abstract

Raman spectra are reported for all the four solid phases of CCl₄, and for three of the four phases of CBr₄. All the spectra have been obtained under hydrostatic or near-hydrostatic conditions, and are shown to be compatible with such structural data as are available, except for CBr₄(I) for which a pseudo-cubic monoclinic structure is proposed. The phases CBr₄(IV) and CCl₄(IV) have spectra compatible with predictions on the basis of the SnI₄ structure type. Carbon tetrachloride photodecomposes under pressure to yield, principally, C₂Cl₆ and some chlorine, whereas CBr₄ gives much bromine together with other (unidentified) products. The CCl₄ photodecomposition can be catalysed by the Cr³⁺ present in the ruby pressure calibrant: this alters the mode of decomposition yielding, principally, chlorine. The liquid-to-solid transition pressure of chlorine is ca. 7 kbar at 293 K, and that of bromine is 5.26 kbar at 308 K.