Vibrational analysis of the low-temperature modification of the chloride spinels : Li2MCl4 (M = Fe, Co and Cr)

Abstract

A vibrational analysis is carried out for the low-temperature modification of the chloride spinals: Li 2MC1 4 (M = Fe, Co and Cr), with special emphasis on the theoretical interpretation and representation of the vibrational modes found. Infrared spectra were recorded at room temperature and 80 Kin the mid-infrared (1000 cm- 1 to 500 cm-1 ) and far infrared (500 cm- 1 to 100 cm- 1 ) regions. Raman spectra could not be obtained, due to sample burning. Several theoretical methods are explored for extending structure information (X-ray crystallographic data) to the interpretation of vibrational spectra. Starting with a factor group correlation, the theory is extended with the aid of a line group analysis, stretching-mode analysis, and finally a vibrational analysis, implemented by allowing every atom three degrees of vibrational freedom. The vibrational behaviour of the lithium atoms in the tetrahedral positions is resolved, but the assignment of the rest of the spectra was not feasible, due to insufficient data. Special emphasis was placed on graphical representations using the three-dimensional stereographic method. The possibility that these representations may greatly assist the understanding of crystal structures and the interpretation of vibrational spectra, is explored. Crystal structures and vibrations are represented, and a method for viewing and obtaining these representations are discussed.