HARD MODE SPECTROSCOPY - THE CONCEPT AND APPLICATIONS

Hard modes are, in the context of this review, optically active phonon s which show systematic changes of their Raman and!or IR spectra when the structural properties of a material are changed (e.g. by heating, application of pressure or chemical reactions). As the characteristic length of high-frequency phonons is very short (the Ornstein-Zernike c orrelation length) the structural variations are measured on an atomic scale. This feature is a great advantage for the analysis of heteroge neous materials, e.g. exsolution pattern, disordered systems. The inte rpretation of frequency shifts, variations of the intensities and line width of optical spectra is largely based on symmetry arguments which show that the renormalization of phonon spectra is, in most cases, pr oportional to AQ(2) + BQ(4), where Q is a structural order parameter a nd A,B are numerical constants. Recipes for the analysis of phonon spe ctra including the use of reference spectra, profile analysis and the application of spectral autocorrelation functions are discussed. In th e case of powder IR spectra the effect of the embedding matrix in a po wder pellet has to be analysed. A simple approach to the ''effective m edium theory'' is reviewed. The effect of short-range structural order on the phonon spectra is discussed using the phase transitions in lea d phosphate (Pb-3(PO4)(2)), titanite CaTiSiO5 and lead scandium tantal ate PbSc0.5Ta0.5O3.